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

Many years ago now, Ronald Brakels was born in Toowoomba. He first rose to international prominence when his township took up a collection to send him to Japan, which was the furthest they could manage with the money they raised. He became passionately interested in environmental matters upon his return to Australia when the local Mayor met him at the airport and explained it was far too dangerous for him to return to Toowoomba on account of climate change and mutant attack goats. Ronald then moved to a property in the Adelaide Hills where he now lives with his horse, Tonto 23.


  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!

  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.

  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.

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