Solar Hot Water Diverters Beat Batteries On Energy Storage

battery vs hot water cylinder

If you want to store your solar energy, should you put it in a battery or in your hot water cylinder?


Update: Compare all the hot water diverters available in Australia here.

How A Diverter Can Give You Solar Hot Water And Store Energy At A Lower Cost Than Batteries

A solar hot water diverter is an electronic device that sends surplus electricity from your rooftop solar to your electric hot water cylinder.  This reduces, or even eliminates, the need to use grid electricity to heat water.  Under the right circumstances diverters can save you money and allow you to store energy at a cost lower than batteries.

Diverters work with standard electric hot water systems that consist of a storage tank with an electric heating element inside.  These are common in Australia, though Victorians and Western Australians generally use gas hot water systems.

Whether or not a diverter will pay for itself depends on a range of factors.  These include a household’s hot water consumption, location, feed-in tariff, controlled load or off-peak rates, retail electricity plan, and the amount of surplus electricity produced by a home’s rooftop solar system.

We have now published a SolarQuotes Diverter Comparison Table giving the details of all the diverters we have information for.  This article is about how they work in general and how much money they may save households in locations throughout Australia.

Diverters Allow Solar Energy To Be Stored As Heat

If you know what a solar hot water diverter is, there is a good chance you’ve heard they allow your hot water system to work as a battery.

This isn’t true.

Batteries and hot water systems are completely different things.  With batteries electricity goes in and electricity comes out.  With a hot water system electricity goes in and hot water comes out.  If you put electricity into your battery system and hot water comes out, then something has gone seriously wrong.  And if electricity came out of my hot water system, I’d be shocked.

But even though they are quite different things, both batteries and hot water systems are able to store energy.  Batteries are able to output the same sort of energy that went into them while hot water systems are only able to store energy as heat.

Diverters Use Grid Power If There Isn’t Enough Surplus Solar

If a solar system is large enough, or hot water use low enough, it is possible for a diverter to make a hot water system entirely solar powered.  But in winter, when solar output is low and hot water consumption is high, many households will find they won’t have enough surplus solar electricity to keep their water hot during extended periods of cold and cloudy weather.  To get around this problem diverters will allow grid electricity to boost the system when required.

Diverted Surplus Solar Can Only Be Used For Electric Heating

Diverters are capable of sending even small amounts of surplus solar electricity to electric heating elements in a form they can use effectively.  But because this power isn’t provided in the usual steady fashion it can’t be used with anything that has pumps, motors, or electronics and it’s a really bad idea to try it – just to see if I am right.

As a result, they can’t be used with heat pump hot water systems, but they can be used to give a boost to solar hot water systems.

While their most common use is powering hot water system heating elements, they can also be used for home, spa, or pool heating.  Depending on the diverter, it is possible for them to send surplus solar power to more than one heating element.

If you want to see what an electric element looks like, don’t open up your hot water system, just look inside your electric jug.  It’s that metal coil at the bottom.  Unless yours is weird, the element will contain an alloy of nickle and chromium called nichrome that has 100 times the resistance of copper1.  The nichrome is surrounded by a ceramic insulator and the whole lot is inside a stainless steel outer layer, which is what you can see.  An interesting fact is the ceramic insulator actually breaks when they turn it into a coil, but it still does its job.  The nichrome conductor never comes into contact with the stainless steel exterior at least 9 times out of 10.

Diverters May Have Other Features

While a diverter’s main function is to supply surplus solar electricity to electric hot water heaters, they many come with other features.  These can include supplying normal household current to devices that aren’t heating elements once there is enough surplus solar energy to power them.  This is often called threshold power, as it is only provided once surplus solar reaches a set amount.

Electric Hot Water Systems Have Not Been Phased Out

In 2010 it was decided to phase out standard electric hot water systems because of the large amount of energy they use, but this has mostly been dropped.  Probably because someone pointed out to politicians that hot water systems are still made in Australia.  There are some building codes and incentives in place to encourage people to install something else, but it is usually still possible to install a conventional electric hot water system if that is what you want.

How Hot Water Diverters Work

The amount of electricity produced by a solar system through the day varies as the sun rises and falls and because of changes in the weather, especially cloud cover.  In addition, the amount of electricity consumed by a household can dramatically rise and fall.  This normally results in large variations in the surplus solar electricity available.  Often there won’t be enough to give a hot water element the power it was designed to use.

The smallest hot water elements draw 1.8 kilowatts, but most are larger and draw 2.4, 3.6, or 4.8 kilowatts.  However, all the diverters I am aware of only work with elements that are 3.6 kilowatts or less.  As solar systems will generally deliver significantly less power than the maximum capacity of their solar panels, even on a clear day a 5 kilowatt solar system may be unable to give enough power for a 3.6 kilowatt element by itself and when it is overcast it will be impossible.

Diverters get around this problem by taking any surplus solar power that’s available over about 100 watts and sending it to the hot water system in a form it can use. The standard way to do this is by chopping the AC power waveform into pieces.

So if a solar system was producing 1.2 kilowatts of surplus electricity it could give a 3.6 kilowatt element the current it is designed to use for one-third of the time. This will result in the element heating water at one-third the rate it would if it was receiving the full 3.6 kilowatts constantly.

Update 22 August: I unfortunately gave a wrong explanation of how diverters work and have removed the incorrect information to prevent it confusing people.  I apologize for getting it wrong.

Efficiency And Environmental Benefit

Diverters are not 100% efficient, but if they are properly designed they should be pretty close to it and lose a very small amount of surplus solar electricity.  Because there will be a small amount of energy loss, at the moment using a diverter is slightly worse for the environment than simply sending the surplus electricity into the grid for a feed-in tariff, but the effect won’t be large and will likely change in the future as solar generation increases and we need storage to prevent clean solar energy going to waste.

Controlled Tariffs And Time-Of-Use Tariffs

To determine how much money a diverter can save a household, it is necessary to look at how much grid electricity used for heating water costs.  In Queensland, NSW, and South Australia it is common for people to use a controlled load tariff, also called an economy tariff, to power the water heater.  This will supply electricity to the hot water system at a rate that is less than the normal tariff, but is only available for a limited period each day.  Controlled load tariffs vary according to location and retailer plan, but prices of 17 cents or more per kilowatt-hour are now common.

It is possible to use a diverter and keep a hot water system on a controlled load tariff, but this will need extra hardware that will increase the cost of installation.

People who have smart meters can also use a time-of-use tariff that provides low-cost off-peak electricity from as little as 12 cents a kilowatt-hour in Sydney.  Hot water systems will generally need a timer to take proper advantage of this.  A suitable timer can cost as little as $20, but paying someone to install it can be considerably more.

Hot Water Consumption

Around one-quarter of electricity consumed by Australian households goes towards heating water if they have a standard electric hot water system.

I’ve looked at various figures available for hot water use in Australia and I’m afraid they just don’t add up.  I can only conclude there are a significant number of people out there who don’t shower every day.

The amount of hot water consumed by households will vary significantly depending upon how many people live there, location, time of year, personal habits and dedication to hygiene.  Fortunately, it is very easy for people whose hot water systems are a controlled load to find out how much electricity they use to heat water.  All they have to do is look at their electricity bills and it will be right there.

If your hot water system isn’t a controlled load then working out how much electricity it uses becomes tricky.  You could install energy monitoring equipment to find out the exact amount, or if you are like me and morally opposed to things that need effort, you could just use the following rule of thumb:

  • 2 person households use around 4 kilowatt-hours a day for heating water.
  • 3 to 4 person households use around 5 kilowatt-hours.
  • 5+ person households use around 6 kilowatt-hours.

Reduce by around 1 kilowatt-hour a day if you are in Brisbane or Perth and by more if you live somewhere hotter.  In summer the amount may be only half as much, while in winter it may be 50% more.

Solar System Size

If you are going to install a diverter it is a good idea to have a solar system that produces enough surplus solar electricity to power the hot water system most of the time.  The good news is, if you have a 5 kilowatt solar system and normal electricity consumption, then your solar system is likely to produce enough surplus power on average to heat your water even in the middle of winter, provided you live on the mainland.

This does not mean you will never need to use grid electricity for water heating, as there are likely to be periods of particularly bad weather and/or high hot water consumption that will make it necessary, but it does mean grid electricity usage is likely to be minimal.

In Tasmania, low solar output and high hot water use in winter are likely to require regular use of grid electricity in winter for homes with 5 kilowatt solar systems.

Savings Per Year By Location

The amount of money saved by a diverter for each kilowatt-hour of surplus solar electricity it uses to heat water equals the cost of the grid electricity that would have been used minus the feed-in tariff that would have been received.

This amount is easy to determine in rural Queensland because all the prices are fixed.  The cost of the cheapest controlled tariff is 17.4 cents per kilowatt-hour while the solar feed-in tariff is 10.1 cents a kilowatt-hour.  This means each kilowatt-hour of surplus solar electricity a diverter uses to heat water will save 7.3 cents, before accounting for efficiency losses.  If the diverter is 99% efficient then 7.2 cents would be saved.

If a rural Queensland household used 1,500 kilowatt-hours a year for heating water which was entirely provided by a diverter, they would save $110 a year, ignoring small efficiency losses.

I have estimated the annual savings from installing a diverter in Australian capitals, using the following, reasonably favorable, assumptions:

  • Households use 1,500 kilowatt-hours a year heating water.
  • Where applicable, the lowest cost electricity plan SolarQuotes electricity price comparison tool provides for a household that uses 4,500 kilowatt-hours of grid electricity and exports 5,000 kilowatt-hours of solar electricity a year is used.
  • Where available, controlled load rates are paid for heating water.
  • After a diverter is installed hot water systems are entirely powered by solar electricity — even in Hobart.
  • Any economic benefit that may result from other diverter features, such as threshold power, are ignored.
  • Diverter efficiency losses are ignored.

Under these conditions, I get the following savings on electricity bills per year:

  • Adelaide:  $87
  • Brisbane:  $116
  • Canberra:  -$17
  • Darwin:   $0
  • Hobart:   $255
  • Melbourne:  $72
  • Perth:  $291
  • Sydney:  -$21

Savings by Capital

As you can see, under the conditions given, installing a diverter in Canberra or Sydney loses money, while the greatest benefit is in Perth.  However, due to their reliance on gas hot water, standard electric hot water systems in Perth are uncommon.

There are also large savings to be had in Hobart.  Unfortunately, unless a household there has an unusually large solar system, the savings are likely to be less due to using grid electricity for boosting during winter.

Diverter Price And Payback Period

Diverters vary in likely installed cost, but if I assume one can be installed in Perth for $1,500 then it would have a simple payback period of 5 years and 2 months.  This is longer than 5 years, which is the maximum warranty period for any diverters I am aware of.

But, if the diverter lasts for 3 years beyond its warranty and runs without a problem for 8 years, then in Perth it would give the same return as investing the $1,500 at 5.6% for 8 years.

What To Consider Before Getting A Diverter

The potential savings or losses that can result from installing a solar hot water diverter are greatly affected by location, judging from the results I obtained above.  But they are also very dependent upon a household’s individual characteristics, which makes it very important to consider hot water consumption and surplus solar electricity generation when determining if a diverter is likely to pay for itself.

As I have previously mentioned, even with optimistic assumptions, a battery system such as a Powerwall 2 cannot pay for itself anywhere in Australia at this time.  However, a diverter in Perth, and potentially other locations under the right conditions, can give a better return that batteries right now.

But just because diverters can give a better return than batteries, it doesn’t mean they are the best option available.  Before installing a diverter you should first check you wouldn’t be better off spending the money on increasing the size of your solar system or investing in energy efficiency such as a heat pump hot water system, insulation, or simply replacing an ancient refrigerator.


  1. This might make nichrome sound like it has a lot of resistance, but it is still a far better conductor than most non-metallic objects such as wood, cheese, your neighbor’s cat, or your neighbor.
About Ronald Brakels

Joining SolarQuotes in 2015, Ronald has a knack for reading those tediously long documents put out by solar manufacturers and translating their contents into something consumers might find interesting. Master of heavily researched deep-dive blog posts, his relentless consumer advocacy has ruffled more than a few manufacturer's feathers over the years. Read Ronald's full bio.


  1. john mcarthur says

    Hi Ronald

    I ave been reading your posts for some time now. Always nformative.

    I have long thought that electric heaters should be the heater of choice with the affordable solar systems available, and i have always advised my friends considering solar PV systems to over, rather than under design the system.

    However, I have been looking for a reliable, value for money electric heater for my pool. With most solar systems, the largest amount of ‘surplus’ solar energy generated occurs in the October to March tie frame. This fits perfectly with pool heating requirements. Why do you think the electric pool heater hasn’t come back into ‘fashion’?

    • John McArthur: I agree – I did the research and couldn’t find any electric pool heaters. All I could find were some electric spa heaters, but they were not suitable. As per my other comment, I ended up rigging up an array of 1kW titanium fish tank heaters. You can get them in smaller sizes too (500W, 250W, 100W… you name it).

      The particular make I purchased was “Weipro”, via ebay. 40 USD each.

      I rigged up a triangular shaped rack using PVC pipe from bunnings. When spring comes around I take the rig out of storage in my garage and sink it into the pool.

      Of course you still need to rig up smart switches and a control system to switch them on only when excess is available.

      Not recommended for people without a decent knowledge of computers and power electronics.

      And don’t forget safety! Proper electrical insulation. RCD’s. Yada yada. Electricity (and lots of it) + body of water + someone who doesn’t know what they’re doing = danger!

      That said, mine is working well

    • Ronald Brakels says

      Hi John.

      I don’t think the electric pool heater was ever really in fashion in Australia. But then, I’m from Queensland, so what do I know? In colder states they may have been more common. What I do know is that an electric pool heater consumes a massive amount of energy, so they cost a fortune to run. As a result, the only options I see people using these days for pool heating are either energy efficient heat pumps or solar thermal.

      • That’s what I concluded as well but the heat pumps are a bit exy. I have plenty of solar as I designed the system to provide sufficient power to cover my entire needs during the winter months.

      • Ron,

        Heat pumps are expensive mechanical devices (expensive in terms of capital cost, installation cost and repair costs). There’s no doubt they are the most (electrical energy) efficient thing out there (typically between 200% – 400%) more efficient than electric heaters because they scavenge heat from the atmosphere, but that comes at the aforementioned costs.

        For the same excess PV available capacity I could use a heat pump instead of my electric heater array to heat my pool and thereby get it up to temperature sooner in spring (but it wouldn’t be as soon it first seems because I have much less on/off cycling flexibility with a heat pump), but the massive increase in capital and service costs means it simply isn’t worth it.

        I’ll just wait an extra week or three for my electric heaters to bring my pool up to temp.

  2. Better yet, if you’ve got electrical and computer skills, rig up an array of titanium fish tank heaters connected to smart switches and a computer control system that switches them on according to the excess PV capacity available (via data from your bidirectional smart meter – you got one of those fitted right?).

    I have a 10kW PV system on my roof and have rigged up an array of heaters totalling up to ~6kW (if all on at once). This replaces the solar (thermal) pool heating system that had to be removed to make room for my PV array.

    A pool heating system like this that runs off excess PV capacity is ideal because the time of year you want to be heating your pool (spring and autumn) is the time of year that home cooling/heating demand is lowest (and therefore maximal excess PV capacity for pool heating).

  3. Peter Seligman says

    Ronald you’ve let us down! You said “If I supply 15 amperes of current at 240 volts to a 3.6 kilowatt element it will produce 3.6 kilowatts of heat. But if I half the amount of current and supply 7.5 amperes at 240 volts the element won’t produce half as much heat which would 1.8 kilowatts, but only one-quarter as much which is 0.9 kilowatts.”
    If you halve the current you won’t have 240 volts any more – you will have a different resistance element. Refer Georg Simon Ohm, 1827. Or if you have the same resistance element, you wont have 240, but 120 volts. But we know what you mean P= I^2 R

    • Finn Admin says

      Well spotted Peter. Ronald likes to break rules but breaking Ohm’s law is going a bit far!

    • Ronald Brakels says

      Yes, I am sorry. I have let everyone down and made a very large and embarrassing mistake. I have excised the blatantly wrong section of the article so it won’t confuse people.

      Thank you for pointing it out, and I promise to never again dabble in things I don’t understand. Except surgery.

  4. Happy Steve says

    What do you think of the Catch Power Hot water Controller??

    I have fitted this to my 5.3Kw system, It does the same as what you mentioned sending un-used power to the hot water system. It also has a built in program that checks when the last time the hot water system achieved cut out. If it has not achieved cut out over a period of time it switched on the night rate for a top up as required.

    Cost was $990 and Australian made.


  5. Hi Ronald,

    Are you sure about your “Halving Current Quarters Heat Production” comments? By my reckoning, if you halve the current and keep the same voltage, then the power will halve too (P = I x V). The square law comes in when you let the voltage vary and then use P = I x I x R (usually happens when you have a ‘constant current source’ and can set the current so that the voltage has to adjust to suit). This gets complicated if the value of the resistance varies (which it does) but I don’t think this is significant here.

    • Ronald Brakels says

      Yes, I screwed up, I’m afraid. I have removed the offending part of the article so it won’t confuse people — and not at all to cast the evidence of my incompetance into the memory hole so it will never see the light of day again. I’m all about preventing confusion.

  6. Hi Ronald,
    What model diverters have you been considering. I am living off grid and as you can only send a limited amount of current to the batteries, there is often excess power available to send to a hot water system, most of the diverters I have seen are not much better than digital variacs which will reduce the current to the fixed impedance load as required. Problem is that the diverter are inefficient and waste the energy as heat. Keen to see effective diverters that you discuss above. Can you supply some models to review.

    • Ronald Brakels says

      I will do a write up on all the diverters I can get information for soon. I am interested in how efficient they are and how much heat they give off. None of them appear to have much in the way of heat sinks, so presumably they aren’t too bad.

    • Peter Seligman says

      Hi Trevor
      I don’t think you can say diverters are inefficient, they switch the current with minimal voltage drop. I wouldn’t expect more than 1 volt drop at 15 amps which would give an efficiency of 99.6%. Digital variac is a good description and I expect they would all do a good it.

  7. Solar hot water is more than 50% efficient. Why use electricity to do what a hot water system can do more efficiently? Like 5 x more efficient!

    During the winter months you will definitely have to have supplemental energy to heat your water , but I think natural gabs cost much less per MMBTU than electricity will every be from the grid. But if you can afford a 10-15 kW PV array then that is a different story.

    John D

  8. G’day Ronald. Thanks for another informative article. I look forward to reviewing the comparison table when it become available.

    I specifically looked at diverters some time ago now, not only for diverting onto an off peak circuit for water heating, but also to try and drive 7 x large whole of room disused in-slab heating systems. Suffice to say, current offerings in the diverter space lack the capacity to drive the entire load, as typical diverters are only rated up to 3.6kw. Some diverters, have additional connection switching interfaces to switch on additional loads or other relays to activate separate circuits, but all-in-all, the amount of solar that can be diverted is still limited to limits of the primary device. The other limitation, is trying to find diverters that are 3 phase capable and that can auto-switch between phases.

  9. Doesn’t it make more sense to replace the electric hot water system with evacuated tubes and an in tank booster. when the existing electric off peak system expires?

    If on a limited budget, Isn’t it better to add a solar hot water system, rather than solar panels from a macro/global climate change point of view as the coal fired generation for the off peak system has the same emissions even though it is much cheaper and because displacing base load power with renewables is needed to reduce emissions and fossil fuel dependence? (Maybe I’ve conflated two thoughts here). There are two sometimes conflicting goals for many users of renewables, personal financial betterment and global reduction in emissions and actions/investments differ depending on which goal is given priority.

    Also beware of assuming that any manufactured item will give trouble free service for its whole warranty period and that breakdowns are cost free because of the warranty. There is your time at the very least not only in gaining service but also in gaining recognition of the problem being covered by warranty and often a financial cost if they can find a way to exclude the warranty claim. Anyone who has followed car problem forums can see that a warranty does not mean 100% service from 100% of items for 100% of the warranty period, nor that 100% of warranty claims are successful. And often a warranty is only on a pro-rata basis which may mean a financial cost of repairing a nearly warn out inefficient device which is less feature rich than its new replacement would be.

  10. I can see how a diverter might make sense in a jurisdiction where the power for hot water is supplied on a much cheaper tariff. To the best of my knowledge as a 20-year Perth resident, this doesn’t happen here. Most customers are on the flat-price A1 tariff. It’s true that you can use SmartPower (a time-of-use tariff) and time your HWS to draw power at night, but for many consumers who can’t or don’t load shift the hefty price for peak consumption would eat up this benefit.

    So, considering only a case where it costs a flat rate to import power (~30c/kWh), and a PV-equipped consumer gets very little for exporting power (`7c/kWh) my question is this…:

    Why would I buy an expensive HWS diverter instead of a cheap HWS timer? If I shift my HWS load to the middle of the day then surely this helps me maximise self-consumption for the PV power. I figure these basic principles apply:

    – Since my HWS and my toaster use electricity at the same price, I don’t care which device is using my home-made electricity as long as it is consumed somewhere in my house – so switching from one to the other doesn’t change the net instantaneous power that I generate, consume, import or export
    – My HWS will consume a similar amount of power whether I time it for midday or midnight (in fact if I’m in the habit of evening baths then midday makes marginally more sense given that HWS insulation is good but not perfect)

    The only issue I see is that if my PV array is smaller than my HWS element (e.g. a 3kW array and a 4.8 kW element) then I will still be importing some electricity when the heater is on. Switching out the element for a smaller one (1.8 kW or 2.4 kW) and allowing a longer timeframe for heating would make sense if this were the case (assuming I have the kind of HWS that allows this). If a diverter is cheaper than the cost of fitting the timer and replacing the element then maybe I can see a case for it.

    Nonetheless I’m not convinced that the instantaneous second-by-second optimisation is required when at the end of a day all I care about is [Generation + Import = Consumption + Export] and I don’t give a fig for which device uses my solar-harvested electrons!

    • Ken Smith says

      The whole point of a Diverter is to send the exact excess solar to the hot water heater element so that the effective grid export and import is 0.

      In practice this is impossible, but overall a diverter like ‘Paladin ‘ (and I am the software engineer for that – full disclosure) manages less than 5% loses in terms of export overall. It really depends on the consistency of the solar and the variability of the house load. It the induction hob or the clothes iron are running a diverter really shows its’ colours.

      Paladin takes around 3200 readings of the grid exit condition every second and then decides just how much excess solar can be sent to the hot water element every 50th of a second. This is the limit of control as the element must be (should be) switched on the crossing of the AC cycle. Paladin uses a form of PWM that is recalculated every 50th of a second and thus avoids flicker and any subliminal effects that can be problematic with some control methods.

      With very fast readings you can get a good instantaneous picture of what is happening on the house circuit and by using a good quality and high amp rated SSR for longevity and safety – (SSRs get very hot when working close to their limits. Paladin uses a 40 amp SSR for a 16-20 amp job) it is a very effective and robust solution to storage and use of excess solar.

      But to get anything close to decent performance you definitely need high read speeds on the grid exit and this unfortunately needs a wired link to the mains CT. WE have tried wireless and it sucks in comparison. Hard wiring the mains CT can be a pain, but it is worth the effort is outright performance and reliability.

    • John McKenna says

      I think your right here, I have a 10kW system that can not make good use of the diverter.
      I have an installation to divert power called “Catchpower” that is not delivering on what I was told by my installer. To start with the thing was never set up for the right time in the first place and then it was set to divert only when there was full power available, which in winter was never. After getting the timing issue and some other issues sorted out with this and the rest of my solar system, I had it set up to draw full power that is, it was acting like a timer and come on at the middle period of solar production but it drew 4.8kW on the element and that meant I had little power for everything else and was still paying for electricity. There was no happy medium.
      As a final nail in the coffin for the diverter, was the electricity company can turn on the controlled load at any time of the day or night and this power overides the diverter, which usually happens on weekends and midday when the sun is shining. Ergo i get no benefit from the diverter on those days and I then send them cheap electricity and buy it back expensively.

  11. Hi Ronald,

    I am in Brisbane and looking to install a solar system with diverter that is adequate to meet my electricity needs as well as heating needs (especially in winter months). Heating accounts for nearly 40% of the total annual kWH consumption and so must be addressed.

    I have 2 questions:-

    1) I am confused by 2 apparent conflicting paragraphs from your article as to whether a 5 kW PV installed system can achieve my above goal. I quote the 2 ‘conflicting paragraphs’ –

    “As solar systems will generally deliver significantly less power than the maximum capacity of their solar panels, even on a clear day a 5 kilowatt solar system may be unable to give enough power for a 3.6 kilowatt element by itself and when it is overcast it will be impossible.”

    “If you are going to install a diverter it is a good idea to have a solar system that produces enough surplus solar electricity to power the hot water system most of the time. The good news is, if you have a 5 kilowatt solar system and normal electricity consumption, then your solar system is likely to produce enough surplus power on average to heat your water even in the middle of winter, provided you live on the mainland.”

    2) Is it possible to use a solar hot water diverter to channel surplus solar energy from PV to hot water system and still keep a hot water system on a controlled load tariff (Tariff 31)?

    Thanks for the clarification.

    • Ronald Brakels says

      Hi Lisy.

      A 5 kilowatt solar system often won’t be able to provide a 3.6 kilowatt element with the full 3.6 kilowatts it needs to operate using normal electrical current, especially if there are people at home using electricity. As a result it will often also need to use grid electricity to operate, especially on cloudy days.

      But a diverter can send small amounts of surplus solar electricity that are less than 3.6 kilowatts to the element in a form that it can use. This way a household with 5 kilowatts of solar panels can heat their hot water through the day with small amounts of surplus electricity without ever having to use grid power for hot water. The only exception likely to be for periods of particularly cold and cloudy weather in winter.

      In Queensland I am afraid it is not possible to have a hot water system connected to a diverter and and have it on Tariff 31 or 33, at least as far as I understand the rules.

      • Hi Ronald,

        Sorry I don’t quite understand your first paragraph. Are you saying that because you don’t have a diverter, a 5 kW solar system won’t be able to adequately heat up a 3.6 kW heating element HWS because any surplus solar electricity will be sent back to the grid at feed-in tariff rate?

        The story changes once a diverter is installed with the 5 kW solar system because any surplus solar electricity will now be diverted to heat the HWS throughout the day.

        My HWS is the conventional resistive element type; not heat pump. My main concern is whether a 5 kW solar PV system with a solar hot water diverter is sufficient to meet my normal electricity power and heating needs as I don’t want to use expensive tariff (since economy controlled tariff is out of the question) as booster for my HWS.

        • Ronald Brakels says

          Hello Lisy

          There will be no problem heating water if you don’t have a diverter. But, even if you turn your hot water system on in the middle of a sunny day, some grid electricity is likely to be used. And on cloudy days grid electricity will definitely be used.

          If you have a diverter and a 5 kilowatt solar system then you will very rarely need to use grid electricity to heat water. A normal household will have more than enough surplus electricity to heat water — except for extended periods of cold and cloudy weather in winter when some grid electricity will probably be needed.

          • Dear Ronald,

            My husband and I are seriously contemplating to go solar and therefore have invested considerable time educating ourselves on this topic. We have satisfied ourselves about:-
            1) When to invest in solar – now even though STC price has dropped
            2) The need for quality components (PV, inverter and accessories eg.
            diverter etc) – established and proven brand
            3) The need for quality workmanship – pre-qualified contractor
            4) The need for adequate warranties

            Annually we use 4000 kWh of electricity for lighting, cooking and air-con and 2000 kWh for conventional electric HWS. We are at home most of the time.

            Our goals for going solar are to:
            1) Use solar electricity generated to meet ALL our lighting, cooling, cooking power needs (67%) AND heating needs (33%) and only send minimal balance back to grid, hence don’t want to oversize to subsidize the ungrateful grid
            2) Continue to use existing conventional resistive element HWS which is still functioning
            3) When the existing HWS needs replacement, can explore other alternatives to heating like conventional solar thermal on the roof or dual elements conventional HWS (I don’t like heat pump).

            There are however still 2 issues we are struggling with:-
            1) The design of the system which will achieve the above stated goals. Obviously we are after a value for money, quality and efficient solution (eg. panel string optimization or ability to bypass individual cells that are soiled, micro-inverters, PERC panels, timers, panel direction, sizing etc).
            2) Price – as we do have a budget to work with and so that we can calculate the payback period. We would appreciate an indicative price range based on the above so that we are not completely off the mark.

            Once we have cleared these 2 obstacles, we will ask for 3 competing quotes from your site and make then a selection.

            Thank you for your help.

          • Ronald Brakels says

            Hello Lisy

            The good news is, feed-in tariffs increased for most Australians on the 1st of July, so your goal (1) may no longer be so important. Looking at a previous comment, I see you are from Brisbane. Using our electricity plan comparison tool:


            I see there are many plans available that offer 11 cents or more. But note the plans with the highest feed-in tariffs aren’t necessarily the best. Offering a high feed-in tariff can be a trick used to lure people in and then hit them with high charges elsewhere. I’m afraid electricity retailers can be pretty sneaky.

            It is very difficult to say what is the best sized system for you, economically speaking. This is because some people will be happy with a solar system that provides a low 5% return on the money they have invested, while other people won’t be happy unless they get 10% or more. If you want the fastest payback time possible then a smaller system is likely to give you that, but because you pay less per watt for larger systems that might be the best investment for you. It will all depend on just what you are looking for.

            If you go to our Solar 101 Guide for Beginners:


            It gives the following estimates for cost by solar system size:

            3kW: $4,000 – $6,000
            5kW: $6,000 – $9,000
            10kW: $12,000 – $16,000

            Now because the cost of solar has continued to fall, you may be able to find quality systems that are installed for even less than this, but this will suffice as a rough guide for how much a quality system will cost.

      • Hi. Regarding this. Do you know if youre able to have a diverter installed in nsw & still have the hot water system connected to “off peak” for those cloudy cold days where the excess power from solar has not been enough to fully heat the hot water?

  12. Hi Ronald,

    I fully agree with your most recent article about going as big as possible with solar in the present environment but I just realize that I have a big problem – shading.

    My neighbor has 3 gigantic palm trees along the fence and as a result I am told (from looking at the google map) that the max number of panels that can fit on the far end of the roof is around 12 to 15 panels (6 front facing and the other 6 back facing). Also my house is NE facing which doesn’t help. I have also checked the price of removing the trees and it is in the thousands of dollars (before talking to my neighbor about it)!!

    Because of the shading issues, it would seem that the PV system that can be installed would be small and has to be super efficient (allow multi panel orientations AND optimizes output of individual panel as well as panel strings).

    After much research, it would seem that we need:
    1) Enphase Micro-inverter which allows multiple panel orientations and optimizes output of the different panel strings
    2) Sunpower X Series PV (which performs individual cell optimization) or at least Trina or Jinko PV (which uses Maxim Integrated cell string optimization)

    On top of that, to have solar for heating, (because of limited roof space), I would need a thermal solar system (which I have not researched much on). Also I don’t know how to integrate it with my existing conventional resistive element HWS (Hardie Dux brand) which is still working well. Can you give me an indication of the cost of the thermal solar system?

    The above solution seems costly and I am feeling uncertain/frustrated which way to go.

    • Ronald Brakels says

      I don’t know where you are, but in Queensland you can require neighbors to trim or remove trees or bushes above a certain height that shade your property.

      SunPower X-Series panels with microinverters would be the most effective way to get the most out of a limited amount of roofspace. The SunPower panels are very tolerant of shade at the panel level and microinverters mean each panel produces power independently so if one panel is shaded it won’t bring down the output of others. But this approach will be expensive.

      If you are happy with your current electric hot water system you may want to put it on a controlled load tariff, if it isn’t on one already, and put solar panels on your available roof space instead. Then in the future when it needs replacing you could get an efficient heat pump hot water system. Or you could do that now if you are eager to reduce your total electricity use. Using high efficiency panels with a high efficiency heat pump can be more efficient at heating water than a solar thermal hot water system.

      If you do decide to get a solar thermal hot water system it may cost $3,000 to $7,000 and will depend on what type you get, your location, and hot water consumption. A heat pump hot water system is cheaper but less reliable.

      Hope this helps.

      Using panels with panel string optimization is probably the lowest cost way of handling shade. However, they won’t produce as much power per square meter as SunPower X-Series panels. But you could afford more panels and maybe having more shaded panels will turn out to be more cost effective.

      • Peter Seligman says

        I hate to see trees go. It is possible that you could put the panels on your neighbours’ house instead and come to an amicable financial arrangement? That would of course depend on how suitable that house is, how friendly the neighbours are and whether their roof is already covered with PV. Another possibility is to put the panels on your kids’ house, or you parents house, or on any other suitable roof.

      • Thank you Ronald for your helpful response.

        Looks like I have to break up the project (to make my home energy efficient) into 2 parts:

        1) I will just install a high quality solar PV on the limited available roof space first and continue using the existing resistive electric element HWS (which is already on tariff 31).

        2) When the existing electric HWS breaks down, I have 2 options (after research):-

        a) Replace it with a conventional solar thermal system (flat plate thermosiphon collector). Questions – is the solar hot water cylinder fitted with resistive electric element/heat pump so that we can boost it with grid electricity? Can we connect the solar thermal system to the economy tariff 31 if we don’t use solar PV to boost it?

        b) Replace it with Heat Pump HWS – can we connect the Heat Pump HWS to the economy tariff 31 if we don’t use solar PV to boost it?

        For the first part of the project, because of cost consideration, maybe I should ask for 2 quotes from each installer – one with micro-inverter and Sunpower X series PV and another with Solaredge DC optimizers with Trina/Jinko (incorporating Maxim Cell String Optimization). Personally I think micro-inverter is the superior option as it is easier to expand the system later (if the neighbor agrees to cut the trees!!) as there will be no panel mismatch issues. But because of budget and payback consideration, there may have to be some tradeoff.

        For the second part of the project, I will have to ask for quotes for conventional solar thermal HWS vs Heat Pump HWS from 3 installers.

        • Ronald Brakels says

          Hello again Lisy.

          If you are using up the sunny part of your roof with a PV solar system, then there won’t be room for a thermal solar hot water system, unless the trees are removed in the future.

          It is common for solar thermal hot water systems to have an electric heating element to boost the temperature if needed. Often this only needs to be done in winter.

          You can put a heat pump hot water system on tariff 31, but you will want to check it is quiet enough for the noise not to bother you at night, although tariff 31 can also turn on during the day.

          You can install a system with a conventional string inverter and then in the future if more unshaded roof becomes available, you can expand using microinverters.

          • Thank you Ronald for the advice to install using a conventional string inverter for the sunny part of the roof and then use micro-inverters for future expansion. That should bring down the cost of installation for now.

            Just want to know if conventional solar thermal HWS (with electric heating element) can use economy tariff 31 for boosting in winter.

            Thanks so much for your time and great advice.

          • Ronald Brakels says

            There is no problem with putting a solar thermal hot water system om tariff 31 for boosting.

  13. Hi Ronald,

    My husband has proceeded to get quotes from 3 installers via your website. Due to shading issues he will ask for 2 quotes from each installer (one with micro-inverter, the other with DC optimizer and string inverter). Cost aside, the micro-inverter seems a theoretically superior solution (redundancy, scalability, point of failures, safety etc). However practically from what we read it seems that micro-inverter has a higher failure rate due to it being on the rooftop, exposed to the harsh weather. Appreciate your comments as this could become the deal breaker.

    • Ronald Brakels says

      Hello Lisy

      Quality microinverters are reliable. For example, Enphase microinverters are the ones most commonly installed in Australia and have a 10 year warranty, which compares well to the 5 year warranty of a typical string inverter. While they’re not perfect, they are generally considered reliable.

      Some consider microinverters to be much more reliable than string inverters because when one fails the others will keep on operating without a problem. But other people think they just mean there are more things that can break down.

  14. Hello Ronald

    What do you think of converting my instantaneous Gas hot water system to electric storage hot water system with a solar Diverter. I have 6.6kW of panels with a 5kW Inverter with plenty of excess power as we live in QLD. We have 7 people living inside our home at the moment and currently roughly go through 1 gas bottle every 2 months which is about $120 a bottle. What’s your thoughts?

    • Ronald Brakels says

      It’s hard to see how you can lose if your are currently spending around $720 a year on LPG for hot water. An electric hot water system and diverter is likely to pay for itself in a reasonable time frame, especially if you are in the Energex area as the solar feed-in tariff is only 7.8 cents there at the moment. I usually suggest also looking at putting your hot water system on a timer, but with 7 people the odds are better than usual that a diverter will be a good investment. You might need a large hot water system if people are showering at night and/or early in the morning.

    • Hi Ronald,

      I am considering a design for a hot water system that looks like the following:

      If there is no sun (or water in the tank before 40C), I do not want the Diverter to direct mains electricity to heat the electric hot water tank. This is because I would like to install an instant gas hot water system as well. The water outlet from the electric hot water tank will be connected to the water inlet of the instant gas hot water system. Is this design possible ? Have you heard of anyone having such a system ?

      Thanks for your advice

      • This is exactly how many solar thermal water heaters with gas boosters operate. Water is heated directly by the sun & collected in a tank. The tank water feeds an instantaneous gas booster which automatically adjusts it’s output to only heat the water as much as necessary, or not at all if it is already hot enough. Instead of heating the tank water directly with sun, you are heating it indirectly with the sun via PV panels & the diverter, but otherwise it is the same. Provided you are only using surplus solar energy, there is no need to limit the amount you heat the tank. The hotter it gets the less gas (non-renewable fossil fuel) will be used.

  15. Giles Dudley says

    Hi Ronald
    I have a 6 month old installed 6.6kwh panels with a goodwe inverter-3phase
    Currently rely on a gas hot water tank but bill is only $25 per month
    However producing currently circa 40kwh in summer and 25kwh in winter… am in Perth WA.
    Have an Unheated pool
    Therefore looking at best way to maximise excess electricity- either for pool heating/ water for domestic use etc—but understand the price point of batteries not effective. Therefore any suggestions/ recommendations for how to proceed- heat pump/ diverter etc?
    Thanks in advance

    • Ronald Brakels says

      Hi Giles

      If you are paying around $300 a year for gas for your hot water system you are likely to come out ahead by going electric.

      If you get a standard electric hot water system and a diverter that would probably save you over $200 a year. As you’ll need to buy an electric hot water system that would still take a considerable amount of time to pay for itself, but many people would still consider it worthwhile.

      Another option would be to get a standard hot water system and put it on a timer so it switches on during the day. This won’t save as much as a diverter but it will cost less up front as you won’t be paying for a diverter.

      Heat pump hot water systems are very energy efficient and generally always come with a timer feature so you can set them to only turn on during the day. However, their weaknesses are poor reliability and high repair costs. You can invest in a high quality one with a decent warranty, but they will cost a lot up front.

      As for a pool heater, they use so much energy I’d say a heat pump would be the way to go.

  16. Thanks for the article on solar diverters. I live in Canberra and have a 15 kW solar system with microinverters. Our biggest power useage is for in-slab heating, currently on a controlled load meter, so not fed by the solar system. Our solar power is used for everything else, and the excess is exported to the grid. We have gas hot water.

    In summer the in-slab heating uses almost no power. In winter there is not much spare solar power anyway. But for a few months in spring and autumn we still need heating and we do have excess solar power which could be used for heating. e.g in October we used around 20 kWh each day for heating in the daytime, and exported a similar amount to the grid. Would a diverter be worth considering as a way to get the solar power into the slab heating ?

    • G’day Ken,

      I am almost in the same boat as you mate, though I haven’t as yet installed my 20-30Kw of proposed solar at my place here in Kambah. I have a couple of fireplaces, 2 x large sun rooms that help heat the house during the day, and both instant gas hot water and off peak (controlled load 2) hot water also.

      Once solar is installed, I would look at scaling back on gas altogether, if I can make it all work. Saving just the gas supply charge alone would be a bonus.

      My thermostatically (air) controlled floors, when fully activated from a cold start, can consume up over 100Kw per day; rate adapting down to about 60-70Kw per day on average thereafter.

      Like yourself, I have been investigating the various diverters as a means to energize the off peak circuit, and the best I have been able to find so far is the Fronius Ohm Pilot, which appears to have a combined sustained output of 9Kw.

      I am not sure if you can gang these or other diverters together in order to redirect more solar or not. I suspect you would need to have different communications channels between the meterbox and the diverters in order to make any ganging work properly, though I could stand corrected.

    • Ronald Brakels says

      Hi Ken

      Given you have a large solar system and use in-slab heating for a good part of the year, a solar diverter may be able to reduce your grid consumption by perhaps 2,000 kilowatt-hours or more a year. If the cost of grid electricity is 10 cents higher than your solar feed-in tariff then this would save you $200 a year. That makes me think a solar diverter could definitely pay for itself and you’ll be in a good spot if and when solar feed-in tariffs fall. Just note most diverters probably won’t handle heating elements over 4.8 kilowatts.

  17. Frank Tkalcevic says

    Given how putting solar back into the grid is losing value (and may cost us in the future), diverting into my hot water seems to make a lot of sense, at least until batteries become more affordable.

    What’s the latest on this technology (this article was from 2017)?

    I came across this article,, also old – 2018, so the $ calculations are off, but he suggests that diverters may be illegal to connect to the grid. Is there any truth to this?

  18. Hi Ken,
    You may have covered this question or similar elsewhere, so apologies if you have.
    I’m looking to replace an unreliable (either too hot or too cold) on demand gas hot water system with either a rooftop solar unit or a heat pump hot water unit.
    Could you give me some advice here as to which system is is preferable, I’m not overly concerned with cost as I intend to buy a top of the line system whichever way I go, however reliability and servicing would be a consideration.
    I live in NSW on Mid North Coast and get a lot of sun even in winter.
    I have rooftop solar with about 9kw of panels which work well.
    Most of the year there are only 2 of us here so hot water consumption is low.
    The current gas powered system costs us about $400 per year and rising fast.
    We have lots of north facing roof available for solar hot water, which would be the better choice, do you think?

  19. Angela Doyle says

    Hi Ron, we are in NE Victoria, have 6.5kW of solar and a 5kW inverter, 315l electric hot water – we’re about to build a new house and I’m trying to work out the best hot water system.
    – We don’t have gas and I’d like to avoid it (fossil fuels etc)
    – Mostly just 2 of us with family and friends visiting occasionally
    – We’re connected to the grid
    – Increasingly having power outtages, especially in summer, eyeing up batteries as backup
    – It’s VERY quiet where we live (like pretty much SILENT quiet, especially at night) so I’m worried about the noise of a hot water heat pump system
    (Not eligible for the current VIC rebate)
    – In winter I have a hot bath most nights (so the most likely time for our hot water to draw electricity is when the sun isn’t shining?)
    – Solar hot water system expensive up front investment & same issue that need for heating more at night
    – Feed in tariffs dropping

    What about a new electric hot water heater with diverter from PV solar? Thank you very much!

    • Geoff Miell says

      Angela Doyle,
      – It’s VERY quiet where we live (like pretty much SILENT quiet, especially at night) so I’m worried about the noise of a hot water heat pump system

      It depends on what heat pump system you had in mind.

      Sanden claim their units feature a “whisper quiet” operation at 37dB.

      A loudness comparison chart is at:

      I can’t hear my Sanden unit operating much beyond about 15 m line of sight during the day, a bit further at night. Having said that, it’s probably best to avoid having the outdoor unit situated close to a bedroom window.

      – In winter I have a hot bath most nights (so the most likely time for our hot water to draw electricity is when the sun isn’t shining?)

      Ensure you have a big enough storage tank for your daily needs, and then you can heat all your hot water during the day, to take advantage of the energy from your solar-PV system (whenever possible).

      Sanden heat pump units can be programmed to operate during the middle of the day, when it’s usually more energy efficient – see my comments at:

      Note that Sanden units have frost protection. When the ambient air temperature drops below freezing, the unit will operate briefly at intervals to prevent the water from freezing in the insulated recirculation pipes between the outdoor unit and the storage tank.

      Sanden units aren’t cheap, but IMO they are very well made, very energy efficient, and in my area (NSW Blue Mountains – Central West), well supported.

      Check your area is well supported before committing to your choice of system.

      If you are on Facebook, perhaps try:

    • G’day Angela,

      Whilst others may disagree and I might be labelled a heretic, I believe you may be better off going back to a normal resistive water storage unit.

      A bit like yourself, I went through the process of replacing a 400Litre offpeak hotwater service in the middle of last year, and after researching & considering all my options, I ended up replacing it with a $1100 315litre Rheem Stellar “stainless steel” hotwater system, which unlike most other hot water heaters, comes with a 15year unlimited warranty, providing you have the system serviced every 6 years (basically it just needs to be flushed).

      I rationalised my decision down to the fact that the unit was cheaper up front to that of a heat pump, and that according to numerous articles read and the advice of my plumber, heat pumps are notorious for failing, primarily because they have a range of moving parts. I also believe that storage systems are invariably more survivable if you lose power, or if the sun doesn’t shine, for a day or two.

      In the end, I concluded I would be better off putting the money saved into more solar on the roof and selecting a very good diverter to energise the tank and the inslab heaters I have in my house.

      I hope this helps and good luck



      PS I have to declare that part of my plan failed, because a national installation company repudiated the contract entered into; to install the 30KW Sunpower AC 415 system that had been negotiated and deposit taken. Irrespective of this, I am still pleased with the hot water system. The water quality is superb, presumerably because of the stainless steel tank. Cheers PT

    • Ronald Brakels says

      Hi Angela

      I recommend getting the largest solar system you reasonably can. Then you’ll have a choice…

      The lowest cost and most reliable option will be to install a conventional electric hot water system along with a timer. This will allow it to heat the water during the day and with a large solar system there will often be enough solar generation so grid electricity won’t be required. For just two people it’s probably not worth getting a PV hot water diverter.

      If you don’t mind spending more money up front you can get a good quality heat pump hot water system. This will use less energy than a conventional electric hot water system, but has the drawback of not being as reliable because it’s more complex. As others have mentioned, noise can be an issue, but if you run it mostly during the day when solar energy is available, this hopefully won’t be an issue. (Most, but not all, heat pump hot water systems come with timers built in.)

      If the money you save by getting a conventional electric hot water system allows you to afford a larger solar system, go for that option. If you are already getting the largest solar system that will reasonably fit on your roof and are looking for other ways to cut your electricity use, get a heat pump hot water system.

      Provided the hot water system is large enough it will have no problem storing enough hot water to get you through the night or through blackouts that aren’t too lengthy. If you never run out of hot water with your current sized system, then you’ll know that’s large enough for you.

  20. Hi Ron,
    I’m planning on installing solar panels and a battery on a new build. Would it still make sense to also include a diverter for hot water (existing evac-tube hot water system) given that the battery would store any excess power which can then be used to heat the water when required? Seems like I might be unnecessarily doubling up and adding to the cost.

    • Ronald Brakels says

      Sorry for the delay in getting back to you, Eddieosh.

      If you have an evac-tube solar thermal hot water system then it seems likely a diverter would never pay for itself in its lifetime, since you are likely to be using far less electricity for heating water than a typical household. With a battery as well the payback could easily be worse. Generally speaking, hot water diverters pay for families with conventional electric hot water systems and high hot water use.

  21. Hi Ron,

    Just wanted to check on your assumptions in this article re. Sydney savings are still valid here which is based, I assume on a night time/time of use tariff being lower than any daytime feed in tariff resulting in no savings if using a diverter? Due to the reduction in feed in tariffs I suspect that might have changed now?
    I’m doing a major renovation of an inner city house and planning to install solar with it. From everything I’m reading a conventional electric hot water with time of use is likely to be the best solution for us.

    • Ronald Brakels says

      Diverters still give a better return than batteries, but for a normal household a standard electric hot water system with a timer (preferably one you can adjust yourself) is often the most practical option. If your hot water consumption is especially high a PV hot water diverter can quickly pay for itself but for many homes a timer will do. You can also consider a heat pump hot water system if you’re goal is to save energy. But the reliable ones tend to be expensive and the unreliable ones don’t last long.

      • Thanks for the response. I still wanted to check the reasoning in your original article with respect to Sydney not delivering any bill savings which I assumed was because the feed in tariff was higher than an economy/time based billing tariff. Is that still the case today?

        • Ronald Brakels says

          Off-peak rates on time of use tariffs and control loads are more than feed-in tariffs in Sydney at the moment, but the difference may only be about 5 cents, depending on the plan.

  22. Hi Ron,

    Some great information here. I’m building a new home and planning to install 23KW PV and a 400L resistance hot water unit. Would PV diverters be necessary with this PV system? If I reduced the tank to 315L, would it make a difference to requiring diverters? I have four young children and in our old home, the 400L never ran out.

    Furthermore, I intend to install a hot water recirculation system, due to the house being rather long – essentially a single storey home, apart from a basement garage. We prefer not to wait a minute for hot water to arrive as well as wasting it.

    I also looked at a heat pump, however the set up for a heat pump with a hot water recirculation system appears complex. According to the plumber, I will require an additional 50L electric hot water reservoir which will require constant power. I’m not sure if you know of a better way to install this, as I would prefer to go heat pump if possible. I can’t seem to find too much info on heat pump plus hot water recirculation,

  23. Matthew Stevens says

    Hi all, I’m unsure if it’s been covered or not, will a diverter or smart water heater consume all excess electricity? If I have a very large solar system, can I still export, divert and self consume at the same time?

    • Ronald Brakels says

      Hi. A solar PV hot water diverter will send power to the hot water up to the limit the diverter can send and the hot water heating element can accept. Some diverters, such as Catch Power ones, can send up to 4.8 kilowatts, which is the size of the largest hot water heating element. But if the hot water system has a smaller element, such as 3.6 kW or 1.8 kW that the maximum amount of power it will be able to send. So let’s say you have a large solar system producing 8 kW while your home is consuming 2 kW and your hot water system is consuming 4.8 kW. This will leave 1.2 kW for export to the grid.

      • Matthew Stevens says

        Thank you for your reply, if I may ask, would the diverter divert constantly if there is excess electricity or would it switch itself off if the water was hot enough?

        • Ronald Brakels says

          The water heater will switch off when hot enough, just as it normally does when not connected to a PV hot water diverter.

  24. Solar Hot Water Diverters Beat Batteries On Energy Storage
    August 22, 2017 by Ronald Brakels


    Are there any similar articles with the savings to be made in August 2022?

  25. Great article, many thanks.

    I am looking to get a diverter for my planned system, but this line you wrote worried me somewhat: “However, all the diverters I am aware of only work with elements that are 3.6 kilowatts or less.” I have a 4.8kw hot water system.

    I see this must have changed as in a very recent comment you say: “Some diverters, such as Catch Power ones, can send up to 4.8 kilowatts”.
    I was planning to get the Catch so should be OK. It might be worth editing your original article for those people who don’t happen to scan through all the comments.

  26. Peter Olsen says

    You say above that solar diverters are not cost effective in Sydney. I think it is time to update that information. We are installing an 8.36KW system for a 4-person household so there should be plenty of spare power. (Energy Australia will kindly increase our energy prices by 10% when we convert to solar). We will then be paying 22c/kwh for controlled-load-1 power. Current feed-in tariff is 7.6c. So solar will save us 14.4c/kwh for hot water. Our average daily hot water usage is 8.3kwh or 3030kwh per year. If that was all powered by solar it would save us 3030 x 14.4c = $436 pa.

    We are on 3-phase so the only option is the Sunmate which costs $1,397 delivered plus $104.50 for their off-peak relay (The relay can be bought elsewhere for $48, such as plus installation cost by our solar company (minimal). Based on that the system should pay for itself in 4-5 years.

    Note that Sunmate advertises their products without including the GST, in breach of the Trade Practices Act. See

    On your web page at you state that their 3-phase diverter is $1,250. The actual price is $1,375 plus $22 delivery as stated above.

  27. Peter Olsen says

    There is another possible poor-man’s solution for anyone with a relatively big solar system plus an inverter that can be accessed and monitored via a web browser (eg. Sungrow).

    Run a program on a pc that uses screen-scraping to check the web page of the inverter every few minutes and look up the spare power that is being exported to the grid. If it exceeds the size of the hot water element then activate a USB-connected relay to send power to the HWS. The HWS element could be down-sized (eg from 3.6kw to 2.4kw) to increase its likely activation.

    USB relay modules cost around $14 on eBay. That relay would switch a bigger amperage contactor on the switchboard to do the actual switching of the HWS, for example this one costing $48: A used PC can be bought for little money so it can be set up as a dedicated monitor.

    I asked Chatgpt-4 about it and it kindly offered to write all of the Python code for me. (Don’t laugh. I recently asked it to write a program to do a regular search of a sales website for a certain item and send me a messenge via messager whenever it found a new ad listed. It works brilliantly.)

    The system would obviously not be as efficient as a ($1,000) solar diverter, but it would still achieve significant savings for little outlay, especially in summertime.

    Another option would be to run the python script on a mobile phone. It would access the web server on the inverter and obtain the necessary output figure. It would then use IFTTT to turn on a smart switch like those used with Google Nests and Amazon Echos to turn on devices. The smart switch would be located in the meter box and would activate the contactor on the switchboard.

    Once again Chatgpt-4 offered to write all the necessary code and explain how to configure it. The phone would not need a sim card, just wifi. It would be less intrusive than a dedicated PC and also wireless.

  28. I understand that (mostly due to solar) in the three months to the end of March, “Between 9am and 5pm, wholesale electricity prices were negative in South Australia and Victoria, 60 per cent and 55 per cent of the time respectively,” (
    Has anyone heard of any retailers capitalising on this by switching on their customers hot water systems via smart meters, and actually getting paid to take the power? I would definitely sign up to a plan where I was guaranteed, for example, any 4 hrs out of 24. I assume this would allow retailers to charge me the lowest possible price. I’m not an engineer but I would guess that giving retailers some control over demand could allow more solar into the grid.

  29. Without the need for diverted that is.

  30. Hi Ronald, I live in Perth and am installing a 13.2kW system. We have a gas tank water heater (170L) that is getting towards the end of its life and will likely need to be replaced soon. We are a household of 3 and the 170L gas tank heater comfortably does the job. I like the efficiency of a heat pump but our tank is near a bedroom and I don’t like the idea of a noisy hot water system. I was thinking of replacing the gas with a standard resistive electric system and heating the water during the day. There is no controlled load in Perth and on the Synergy midday saver tariff, the overnight tariff is around 22c. Cheapest tariff is between 9am and 3pm at around 8c. Do you think I would be better off just getting a simple timer to turn on at 9am and off at 3pm (using solar or cheap tariff depending on the weather) or a solar diverter?

    • Anthony Bennett says

      Hi Jehann,

      If you can but a Reclaim or Earthworker heat pump they are exceptionally quiet AFAIK.

      Conventional hot water is fine, you may want a smaller element so it runs for more hours but more of those hours will have enough solar yield… that’s the approach for a timer system.

      A diverter will offer finer control and not require an element change

  31. Nigel Pearson says

    Homes with Smart Meters and Off-Peak(*) hot water systems might already be heating with solar power. This year, my retailer (Origin) took over from the distributor (Ausgrid) the times of the day that the heater comes on.

    Today, for example, it was 4:30-5:00 and 11:30-11:55?
    Yesterday, 3:00-3:30 and 12:00-12:45. Tuesday 2:00-2:30 only?
    Sunday, 10:30-10:45 and 12:30-13:00.
    (I only know the times because I installed a battery system,
    with a CT installed on the grid side of my smart meter ! )

    * The distributor actually calls these Controlled Load 1 or 2

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