Will A Solar Hot Water Timer Save Or Lose You Money?

hot water timer

If you have solar, is adding a simple, cheap timer to your electric hot water system worthwhile?

Lately my life has been a lollapalloza of solar electric hot water heating.  I’ve looked at more ways of heating water using solar electricity than you can poke a stick at.  Or at least I have if your poking arm is weak and flabby and easily fatigued.

Recently I went into the details of using a relay with a Fronius inverter so it will turn on a hot water system when there is enough surplus solar electricity to power it.  I also gave a run down on solar hot water diverters in general, which are devices that will take even small amounts of surplus solar power and send it to a hot water system in a form they can use.  And I delved into the secrets of the Sun Flux hot water controller that uses it own dedicated solar panels to provide power for heating water.

Soon I will put several brands of diverters into a little Thunderdome and make them fight to see which one will come out on top. But for now I am going to describe the simplest and cheapest way to heat water using solar electricity, which is to use a timer so it switches on during the day, hopefully when there is enough surplus solar electricity to run it.

A timer may only set you back around $100 if you have it installed at the same time as your solar or hot water system.  But if you need to get someone to come around just to install the timer it is likely to cost $200 or more.  And you can always pay extra for something fancy, such as a timer you can easily adjust yourself.

A hot water system timer is a great way to save money provided it doesn’t end up costing you money.  The drawback of a timer is it’s almost impossible to avoid using grid electricity at least some of the time.   You will pay for that electricity at your standard rate and not at a lower controlled-load or economy-tariff rate.

But, under the right circumstances, it is possible for a timer to save money.  These circumstances can include:

  • Having a small heating element in your electric hot water system.
  • Having a large solar system.
  • Being out of the house during the day.
  • Living in a location with decent winter solar output.
  • Having a relatively low feed-in tariff.

If enough of these duckies line up in the right way, then you may be able to save money instead of losing it by installing a hot water timer.

Why Not Use Solar Thermal Hot Water?

Recently, as I’ve been rabbiting on about using solar electricity to heat water, some people have said, “What’s wrong with traditional solar thermal hot water?  You know, where water circulates through thermal panels on the roof, the way god intended!  What’s wrong with that?”

Well, it’s not that anything wrong with solar thermal hot water, it’s just there is something right with solar PV, and that is, the price is right.  Solar thermal is a mature technology that hasn’t come down much in cost over the past few years, while solar electricity has.  As a result, many people have concluded PV is more cost-effective for heating water than solar thermal.

But if you want to install solar thermal hot water, please don’t let me stop you.  It does have advantages:

  • Thermal panels need less roof space to heat water than PV panels connected to a standard electric hot water system1
  • Solar thermal works during blackouts.
  • You can add a solar thermal hot water system even if your Distributed Network Service Provider (DNSP) won’t allow you to install any more solar PV.

Timers Are Dumb

In a world full of smart technology and intelligent systems, a simple timer that turns on a hot water system for a set period each day is pretty bloody dumb.  All it can do is turn on the hot water system at a time when you hope there will be enough surplus solar electricity to run it and then turn it off later.  It can’t decide to wait for the sun to come out from behind clouds and it can’t respond to household demand and shut itself off if someone turns on a hairdryer or otherwise increases electricity use.

Hot Water Diverters Are Smart

Solar hot water diverters are not geniuses, but they are very good at slurping up almost all the surplus electricity a solar system produces and sending it to an electric hot water system in a form it can use, so the result is pretty clever.  Sure, it is possible during periods of cloudy weather and high hot water use that solar electricity alone isn’t enough and grid power is needed.  But a diverter is the most effective way to make sure surplus solar electricity goes towards heating water.

Fronius systems are smart

A Fronius relay plus datamanager is a very smart system and capable of turning on a hot water system only when there is enough surplus solar electricity to run it and it can turn itself off if the sun goes behind clouds or household electricity use rises.  It can’t do the diverter’s trick of getting a heating element to use less power than it is designed for.  This means that, all else equal, more grid electricity will be required to heat water than with a diverter, but it is still quite clever.

Even if you use a Fronius inverter that doesn’t have a data manager2, which is the part that is a clever clogs, and use a dumb relay by itself, because it will only turn on when the inverter is producing a set amount of electricity, it still ends up being smarter than just a timer.

Do You Feel Lucky?

So if you have a timer connected to your hot water system, every time it turns on, you are taking a gamble there will be enough surplus solar electricity and your hot water system won’t start consuming expensive grid power.

The Smaller The Hot Water Element The Better

A conventional electric hot water system consists of a storage tank with one, or sometimes two heating elements.  The elements come in the following standard sizes:

  • 1.8 kilowatts
  • 2.4 kilowatts
  • 3.6 kilowatts
  • 4.8 kilowatts

The larger the element the more quickly it will heat water, but the more power it will draw.  If there isn’t enough surplus solar electricity being produced when the timer turns it on, whatever is lacking will be drawn from the grid, so the smaller the element, the better when using a timer.

A 1.8 kilowatt element can heat 250 liters of water from 20 degrees to 60 degrees in 6.5 hours, which is plenty of heating power for a typical household of two or three people, but for a household that is a large user of hot water this may not be enough.

The Bigger The Solar System The Better

The larger your solar system, the more likely it is to produce enough surplus solar electricity during the day to run the hot water system without using grid power.  And you will need a big solar system to be sure of having enough surplus electricity to power even a 1.8 or 2.4 kilowatt heating element.

Unfortunately, for a variety of reasons, rooftop solar rarely produces as much power as the total capacity of its solar panels.  So if you have a north facing solar system with 5 kilowatts of panels, it may not produce more than 4 kilowatts of power even at noon on a clear day.  From mid-morning to mid-afternoon on a cloudless day, such a system might produce from 2 to 4 kilowatts.  Assuming household electricity consumption at that time is minimal, this is only enough to power a 1.8 kilowatt element without using grid electricity.

The situation is improved with a 6.5 kilowatt system, which is around the maximum size that is practical for many households to install.  This could be expected to produce from around 2.6 to 5.2 kilowatts from mid-morning to mid-afternoon — enough for a 2.4 kilowatt element, provided there is no one at home using power.

Larger Solar Systems Help With Cloud Cover

Unless a home’s rooftop solar system is at the edge of being unfeasibly large, it is impossible to avoid a hot water system on a timer using grid power if it switches on during cloudy weather.  While light cloud may cut solar output by 75% or less, very dark cloud can cut it by 90% or more.

If overcast conditions cut output by 80%, then a 5 kilowatt system may only produce from 0.4 to 0.8 kilowatts from mid-morning to mid-afternoon. That’s only enough to supply around one-third the power consumption of a 1.8 kilowatt element, provided the home has no other electricity consumption at that time at all.

The More You Are Away During The Day The Better

If you are not at home during the day then you are not going to be consuming electricity and reducing or eliminating the amount of surplus solar electricity available when your hot water timer switches on.  So if no one is at home during weekdays that is good, and if you are also usually out during the day on weekends too, that’s even better.

The Higher Solar Production In Winter The Better

The colder it is, the more hot water people use, so the higher solar production in winter the better.  Sydney, Brisbane, Perth, and Darwin all have reasonably good winter output and all average 3.25 kilowatt-hours or more per kilowatt of solar panels in June, which is the worst month of the year for solar power.  This is shown below on graphs of monthly output for Sydney and Darwin:

Sydney Solar Output


Darwin Solar Output

Canberra, Adelaide, Melbourne, and Hobart all do much worse.  Here is a graph of monthly average solar output for Canberra:

Canberra Solar Output

As you can see, Canberra only averages 3 kilowatt-hours a day of solar electricity per kilowatt of panels in June, but with its more southerly location and cloudier winters, Adelaide’s June output is even less:

Adelaide Solar Output

In Melbourne the average for June is almost as high as in Adelaide, but Hobart averages under 2 kilowatt-hours a day per kilowatt of solar panels.

Unfortunately, though not unexpectedly, Australian capitals that have the coldest winters and the highest hot water use, have the worst winter solar production.

Controlled Load Or Economy Tariffs Often Can’t Be Used

Most people with an electric hot water system have it on a controlled load or economy tariff.  This allows people to pay less for grid electricity used by their hot water systems, but the power is only available for a limited amount of time each day3.

Hot water systems on a controlled load tariff are not permitted to receive grid electricity from another source.  As a result, most hot water systems cannot be put on a timer to receive solar electricity during the day and be on a controlled load tariff.  An exception to this is double element heaters, which can have one element on a timer and the other on a controlled load tariff.  The element on the controlled load can be set to a lower temperature so most of the heating gets done during the day.

Setting The Timer

Setting a hot water system timer involves striking a balance between minimizing grid electricity use and not running out of hot water.  If your hot water system is on a controlled load tariff and you occasionally run out of hot water, then unless you found these events to be exciting opportunities to have invigorating cold showers, then you will probably want to set your timer so your hot water system will run long enough to heat a full tank of water.

In winter, the water entering a hot water tank might be around 15 degrees4.  To heat a 250 liter tank of 15 degree water to 60 degrees, which is the minimum temperature electric hot water systems must be set to5, will take the following amounts of time:

  • 1.8 kilowatt element — 7.3 hours
  • 2.4 kilowatt element — 5.5 hours
  • 3.6 kilowatt element — 3.7 hours
  • 4.8 kilowatt element — 2.7 hours

For a 315 liter tank increase the times by 25% and for a 400 liter tank increase the times by 60%.

So if you have a 250 liter hot water tank with a 1.8 kilowatt element, you may want to set the timer so it will run for over 7 hours a day.  On most days the hot water system will reach its maximum temperature and switch off well before the 7 hours are up, but on days of high hot water use it may run for the full 7 or more hours.  If the timer was set to switch on at 10:00 am then it should usually reach its maximum temperature by 2:00 pm, but if that is not long enough it will be able to keep heating until past 5:00 pm.

The longer a hot water system is left on, the more likely it is to use some grid electricity to maintain its temperature later in the afternoon.  But that’s a price most people are willing to pay to not have a cold shower.

If you have spent the money to get a timer you can easily adjust yourself, then changing the settings if you need to is not a problem.  But if the timer has to be set by an electrician you’ll want to get it right the first time.

Estimated Savings From Installing A Timer

As you can probably guess, it is pretty difficult to wring savings out of a hot water timer.  I thought about making a 3 dimensional, rotating graph, that would show all the possible permutations of hot water timer phase space and allow people to see exactly under what conditions they could save money.  I thought about it, but I never had any intention of making it.  Instead, I’ll give a variety of examples and use them to estimate the possible savings that may result from using a timer.

I freely admit that in creating the estimates I used some intelligent guesswork.  But don’t worry, if you don’t like that, I’m more than happy to give you unintelligent guess-work.

All the figures for electricity prices and feed-in tariffs are based on actual figures at the locations.

Brisbane Example

If a household in Brisbane has the following characteristics:

  • An average of 4 kilowatt-hours a day is used heating water.
  • No one is home on weekdays.
  • The hot water system heating element is 1.8 kilowatts.
  • Solar output is high enough so 70% of electricity consumed by the hot water system would be solar if put on a timer.
  • Grid electricity costs 27 cents a kilowatt-hour.
  • A solar feed-in tariff of 11 cents a kilowatt-hour
  • A controlled load tariff of 17 cents a kilowatt-hour.

Then under these conditions a hot water timer would save $18 a year.

If the portion of solar electricity used to heat water fell to 62% or less getting a timer would lose the household money.

Sydney Example

In Sydney, a household with the following characteristics:

  • A solar feed-in tariff of 11 cents a kilowatt-hour
  • A controlled load tariff of 9 cents a kilowatt-hour.

Would only lose money, as the feed-in tariff is higher than the cost of the controlled load tariff.  It would make more economic sense to send leave the hot water system on the controlled load tariff and send surplus solar electricity into the grid.

If the cost of the controlled load was twice as high and the household had the following characteristics:

  • An average of 5 kilowatt-hours a day is used heating water.
  • No one is home on weekdays.
  • The hot water system heating element is 1.8 kilowatts.
  • Solar output is high enough so 68% of electricity consumed by the hot water system would be solar if put on a timer.
  •  Grid electricity costs 29 cents a kilowatt-hour.
  • A solar feed-in tariff of 11 cents a kilowatt-hour
  • A controlled load tariff of 18 cents a kilowatt-hour.

Then the household would save $23 a year.

Adelaide Example

If an Adelaide household had the following characteristics:

  • An average of 5 kilowatt-hours a day is used heating water.
  • No one is home on weekdays.
  • The hot water system heating element is 1.8 kilowatts.
  • Solar output is high enough so 66% of electricity consumed by the hot water system would be solar if put on a timer.
  •  Grid electricity costs 32 cents a kilowatt-hour.
  • A solar feed-in tariff of 11 cents a kilowatt-hour
  • A controlled load tariff of 24 cents a kilowatt-hour.

Under these conditions the putting the hot water system on a timer would save $107 a year.

Perth Example

In Perth controlled loads for hot water systems are no longer available.  So for a household with the following characteristics:

  • An average of 4 kilowatt-hours a day is used heating water.
  • No one is home on weekdays.
  • The hot water system heating element is 1.8 kilowatts.
  • Solar output is high enough so 72% of electricity consumed by the hot water system would be solar if put on a timer.
  •  Grid electricity costs 27 cents a kilowatt-hour.
  • The solar feed-in tariff is 7 cents a kilowatt-hour.

Under these circumstances installing a timer would save $210 a year.

Melbourne, Hobart, And Darwin

Due to poor solar output and the existence of controlled load tariffs, a hot water timer is very unlikely to save money in either Melbourne or Hobart.  In Darwin the feed-in tariff is equal to the cost of grid electricity so a timer can’t save money.

A Hot Water Timer Is Unlikely To Pay For Most Households

While it seems possible for a hot water timer to save money in a some locations under favorable circumstances — such as having a small hot water element and not being home on weekdays — it seems likely that putting hot water systems on a timer will lose the majority of Australian households money.

The exception is Perth.  Unlike most of Australia, controlled load tariffs are not available in Western Australia.  This means there is no danger of losing money by putting a hot water system on a timer.  I would say it is worthwhile for anyone in Perth who is getting rooftop solar installed to have their electric hot water system put on a timer at the same time.  But very few people have electric hot water systems in Perth, as most homes there use natural gas hot water.


  1. But this may not be the case with a high-efficiency heat pump hot water system.
  2. These versions are known as “Fronius light”.
  3. Controlled load tariffs are usually provided very late at night and very early in the morning, but in Queensland they may also be supplied in the middle of the day when solar output is high.
  4. A cold water temperature of 15 degrees is low for Australia, but definitely possible in the southeast.  Water that comes from a rainwater tank can be as low as zero degrees and water that refuses to come from a rainwater tank can be below zero degrees.
  5. The minimum temperature of 60 degrees it to prevent the growth of dangerous microorganisms such as Legionella.  To keep them under control, a hot water system should reach a minimum of 60 degrees at least once every three days.  So if your hot water tank fails to get hot for days on end, you have ridden into the danger zone.
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. Peter Woods says

    As far as the hot water is concerned putting it on solar is a good option as it gets rid of the metering charges for the off peak in NSW, we mostly install a time clock although there are issues with legionella if the tank stands with warm water overnight, same with standard solar hot water hence it is not advisable to put isolators on solar hot water to turn them on to the grid if they have no sun for and are not hot enough

  2. You forgot to mention that environmentally it would be better to send the excess electricity to the grid due to the thermal losses associated with hot water storage.

  3. What about evacuated tube systems? I was under the impression they were the most efficient for heating water.

    • Ronald Brakels says

      Going by total roof area taken up by collectors that are laid flat, flat plate thermal solar can be over 80% efficient at transferring the energy in sunlight to water as heat, while evacuated glass are a little better than half that. But evacuated tubes lose much less heat to the environment and so perform better in colder climates. This makes either type more efficient that using PV panels to power a standard hot water system that used electric resistance heating elements. But efficient PV panels plus an efficient heat pump can have even higher efficiency. (I’ll probably write an article on this at some point.)

      • Thanks. I was worried because we have just invested in a sunpower 6.23 system fronious inverter 5kw and an apricus evacuated tube system. Does this sound like a good combo?

        • Ronald Brakels says

          That sounds like a great way to slash your electricity bills and reduce greenhouse gas emissions. With the 10 year warranty on both the Fronius inverter and Apricus solar thermal hot water system, I expect they’ll give you many years of problem free operation.

  4. You mention that “a hot water timer is very unlikely to save money in either Melbourne or Hobart”.

    I’m in Hobart and I’m about to get solar installed. I’ve ordered a timer to be put on my hot water, and I’m going to change to a ToU tariff. In Tasmania peak times are between 7am-10am and 4pm-9pm AEST weekdays. Off peak (other times and all weekend) pricing is actually cheaper by 2 cents than the conventional “HydroHeat” tariff.

    The hot water cylinder is 160L and a 2.4kW element, so using your calculations that is 3.52 hours to heat the water from 15 degrees.

    Because nobody is at home during the week, we plan on setting the hot water timer to run daily from 11am-4pm with the hope of maximizing the solar to divert excess to it, our standby consumption is relatively low (300W).

    We’ll also put in the timer to switch on daily between the hours of 3am-7am (offpeak time) in case the thermostat has dropped overnight for morning showers and use. In winter we can just bump the thermostat up which may result in the water staying warmer for longer, but also potentially taking longer to heat (more grid power).

    So it still can be effective using a hot water timer in Tasmania using the ToU tariff. Now to pre-heat my home during off peak using my heat pump…. 🙂

  5. Having just upgraded our system to well exceed our demand (4x) I can say with confidence the key to avoiding any of these decisions is correct system sizing from the outset. If you size for %100 demand offset on the shortest darkest winter day, the rest of the year will be taken care of, i.e. if a 5KW would meet your yearly average, install 10KW. Prices have never been cheaper and no-one really knows how long the rebates will be available for. My two cents worth….

  6. Bret Busby says

    In the above article, is “The situation is improved with a 6.5 kilowatt system, which is around the maximum size that is practical for many households to install.”

    I have read reports that the extreme right wing senator, who does not believe in solar power having any advantage,has had installed on his own house, a 12kW capacity domestic rooftop photovoltaic system (although, being a member of the federal parliament, and thence, immune from any laws in Australia, he is probably allowed to get away with doing that).

    When we got our domestic rooftop photovoltaic systems installed (we have two, due to the nature of our roof), a couple of years ago, we were prohibited by the state regulatory body (which comprises of the coal fuelled electricity companies) that controlled authorisations for getting rooftop photovoltaic systems installed in WA, from getting any more than 5kW of generating capacity, installed, as any more than 5kW would significantly disrupt the already unstable electricity grid.

    And, in WA, by the same reasoning and regulatory authority, in having a 5kW generating capacity, domestic rooftop photovoltaic system, we are apparently banned from having any associated electricity storage batteries.

    Can you please publish a comparative table that states the maximum allowable generating capacity per household, and, associated storage batteries, for domestic rooftop photovoltaic systems. for each state/metropolitan area?

    Thank you in anticipation.

  7. Pat Comerford says

    After looking at all the options for heating my HWS that you have listed in your article I believe I have installed the perfect combination. To install any kind of separate timer, diverter etc it will be very difficult to achieve any kind of acceptable ROI. So now I have my Powerwall 2 installed and up and running I have had fitted a $18 digital fully programmable seven day timer bought on eBay from China ( very nice little unit). My 3kw inverter coupled with the 5kw of available power from the PW 2 inverter and my timer set for two hours on from 11.30 to 13.30 hrs heats my 315lt tank with single 3.6kw element (takes just over an hour most days so far) has maintained my water temp at around 60c for the last couple of weeks with zero grid power being drawn. I was getting the Battery for grid independence anyway and being able to run the HWS for an extra investment of $18 is the icing on the cake.

  8. I previously had an instantaneous gas hot water system which required delivered gas bottles. Three years ago our cost was $140 bottle hire fee per year + 5 bottles / year at $145 a bottle. Total Gas cost per year $865. Electricity was $500-$600 per quarter so let’s say $2k/year.

    I decided on a Solaredge 5kw inverter + optimisers with Qcells 6.5KW panels + largest Sanden heat pump hot water which only runs between 7am and 5pm. I have not run out of hot water yet with 2 adults and 2 kids. I have some panels east facing, some north and some west to maximise the generation at the start and end of the day. In summer on a cloudless day I can generate in excess of 40kwh. In winter I don’t do as well as strictly north facing panels but I use more energy in summer due to aircon so it suits me fine. This is in Brisbane.

    My total energy cost is around $700 to $800 per year now. I am saving $2k / year which gives me around a 5 year payback for the panels and heat pump. The saving is probably more now because we installed 3 years ago.

    I have not looked back.

  9. Aaron Murphy says

    Is it legal to have your HWS connected to both the general tariff and an economy tariff, but only one at a time and have a changeover switch to pick the tariff you wish use? So if you know its going to be an overcast day you can switch it to the economy tariff?

    • Ronald Brakels says

      As far as I am aware, it is not permitted to have an appliance that is on a economy tariff (or controlled load) connected to the grid in any other way. Not only do the grid operators want devices on economy tariffs to use electricity when demand for grid electricity is low, they also want it to be impossible for those devices to use grid electricity when demand is high.

    • Finn Peacock says

      I’ve been told it is allowed in Tasmania.

      It is explicitly prohibited in QLD.

      My interpretation of the rules in other states/territories is that they do not allow it either.

  10. Erik Christiansen says

    Feeding raw solar power to a HWS element is dreadfully inefficient, and as the article describes, is only feasible if ” there is no one at home using power.” I.e. it wipes out the total output of any permissible grid connected PV array.

    Solarquote’s distrust of heat pumps is based on some bad experience in the past, if I recall correctly, so it is perhaps worth noting that:

    + A heat pump is just air conditioner components running in “reverse cycle” mode. (So throw out all air conditioners too?)

    + Buy a heat pump with a Grundfoss or equivalent quality compressor, and there seems no reason why it shouldn’t be as durable as a good air conditioner.

    + With 3.5 to 4 kW of water heating for every 1 kW of PV power consumed,
    it is no longer necessary to camp out at the in-laws whenever you need to heat water. Just 1 kW creamed off the top of peak array output from 10 a.m. to 2 p.m. is hardly noticeable on an adequate PV array, and 16 kWh would boil most HWS.

    My preference is a heat pump (possibly the “Bolt On”, as I can mount it in the roof space, to refrigerate up there in summer – thus reducing aircon load below) I’m also planning on an in-flue water jacket on the wood heater, for winter water heating. Even with the northern roof at 30 degrees, winter PV output is well down, but the wood heater boost fixes that. (Using solar energy stored in hard little brown consumable storage units which also keep me warm when I cut and split them.)

    Cooling the roof space with the same energy input which heats the water is quite appealing. In summer there’ll be enough hear coming through the roof, but with an insulation blanket under the steel decking, I’ll fit a soffit fan or two, for control of temperature up there. (I bought two crates of Papst 24Vdc fans cheaply, so have enough for replacements for many years.)

  11. Thanks Ronald, this effectively answers the question I posed on another thread… in short a timer does pay in Perth due to the absence of any cheaper controlled load / HWS tariff.

    It doesn’t bother me that patches of cloud might cause the HWS to draw precious grid power. If it was heating at night it’d be drawing precious grid power anyway. When the price of import power doesn’t change and it vastly exceeds the export price, then the HWS tail shouldn’t wag the PV dog. What I care about is the PV power being self-consumed as much as possible. A timed HWS load fits this need pretty well – there’s only so much vacuuming one can do at mid-day…

  12. Dennis Stanford says

    Living in Cairns we have a 3 bedroom house and a separate Bungalow with bathroom,living and it’s own kitchen. We placed the 2 X 250 litre HWS we already owned side by side. The first tank has a 1.8 wattage element pre heating the water on timer, this then feeds the second tank on Tariff 31 which then supplies both buildings. Not sure of the savings but have more than enough hot water.

  13. Jack Wallace says

    Previous comment apparently lost the cyberspace. —>

    Best hotwater system is a roll of cheap polypipe sealed in a box lined with silverfoil with a glass top ~ all run through an 2nd-hand insulated hot-water tank in the laundry. I’ve seen such a unit bring water up to nearly boiling even on a winter’s day in Victoria. All you need is mains pressure or a 12-foot head from a tank and a safety-valve. Takes a day to put together and casts a coupla hundred dollars…..Too easy.

  14. Anthony Page says

    Can anyone tell me where this specific guideline comes from?

    “growth of dangerous microorganisms such as Legionella. To keep them under control, a hot water system should reach a minimum of 60 degrees at least once every three days.

  15. James Kalantzis says

    Hi, thanks for this informative post. So how long will it take to heat a 125 litre tank that is 3.6kw?

    Also, I have a high FiT 22c, so am I better off putting my timer to heat up my tank during off peak hours (15 cents p/kWh) so that I’m getting more back from exporting? I guess if my FiT drops below 15c then I’m better off using my 10kw solar system to heat up my hot water?

    Am I correct or off the mark?


    • Ronald Brakels says

      Hi James

      Assuming the water is at 15 degrees and is heated to 65 degrees it will take a 3.6 kilowatt element just over two hours and it will require 7.3 kilowatt-hours of electricity. But it will normally only be that cold if the hot water system has been turned off for a few days. If the water starts at 40 degrees it will only take half as long and use half as much energy.

      One advantage of having a small tank with a large element is you can switch it on and after 10 minutes there will be enough hot water at the top of the tank to have a shower even though most of the water in the system will still be cold. Fortunately, your 10 kilowatt system should have no problem meeting its power draw on most days if you time it to turn on around noon. But you are quite right that you are better off if it uses off-peak power instead as that is less than your solar feed-in tariff.

      Most people with large solar systems are better off on a standard tariff and paying a flat rate for electricity rather than having a time-of-use tariff, so if a lot of the grid electricity you use is during the peak period you may want look into changing.

      • James Kalantzis says

        Thanks for that post. Answered all my questions and concerns.

        My plan is to maximise self consumption during the day and remain on TOU tariff. Peak is only 5pm-9pm for 3 months in winter and 2pm-8pm for 4 months of summer then other times are shoulder and off peak (15c and 25c respectively) not to mention weekends are only shoulder and off peak too.

        So if it all goes to plan, maximising my self consumption and heating my hot water only between 11pm-7am will mean my peak consumption usage should be very low as the only thing that contributes to high electricity usage during peak hours is mainly the hot water system. The only thing that worries me is having enough hot water in the 125l tank for showers for my family of 4 (2 young children) but knowing I can turn the system on for 15 minutes manually is a great relief.

        Whilst the hours of peak in summer are long, because I have a large system, I should be able to cover most of my electricity costs during the day until at least 4pm-5pm using solar meaning peak will only effectively be 4 or 5pm to 8pm.

        I guess if I’m seeing my usage still high between 5pm-9pm I can switch to single rate tariff (if Ausgrid And origin allows me) And pay the 31c p/kWh.

  16. Matthew Tatten says

    What would you recommend: A fronious Ohmpilot, or an IStore hot water system?

  17. I am currently building/putting solar hydronic heating into my house on nth coast nsw
    have 2 2mxx1m water collectors feeding to older 250ltre storage tank on ground
    tank on ground is heated via 12m copper coil heat exchanger in tank.water pumped through this to panels on roof
    hot water from tank pumped through fan assisted (fans cost 1.25c ea per day) converted oil column heaters.
    warm air is being delivered but needs to have hotter water. mostly getting 38-40c in tank. sealed system so no human contact. feel it needs to boost to 60c
    tank can lose 3-8c over night.
    was thinking of plugging in an electric element for a few hours per day
    I have 4.2 solar system
    your ideas and comments would be greatly appreciated

    • Ronald Brakels says

      A solar diverter can be used to send surplus solar energy that would normally go to the grid to a resistive load such as a heating element. So installing one of them is an option. But hydronic heating uses a lot of energy and your 4.2 kilowatt solar system may may not produce a lot of surplus energy in winter. You can look at your winter electricity bills to see how many kilowatt-hours your are getting a feed-in tariff for in the coldest months. That will give you an idea of how much energy can be sent to heat your floor.

      A simpler approach would be to use a timer so the heating element is only switched on during the day when solar power can contribute to its energy consumption.

  18. Hi Ronald, great site btw. I recently installed a Fronius inverter with stiebel eltron 302h HWS + smart meter. The inverter + smart meter basically overides the HWS during high PV export to “overheat” the water to get more hot water out of it before it requires standard reheating (so like a thermal battery). Unfortunately its not enough to prevent the HWS kicking in to heat back to 60 degrees following the last shower for the evening. It will heat all night and switch off just before sunrise. Sun comes up, PV export kicks in, it overrides and over heats again past the factory 60 degrees and so forth. You mentioned the smart meter switching off the HWS during low PV (ie at night / cloudy day) – my understanding is it can’t switch off the standard heat cycle (heating to 60 degrees) but rather it switches off the override cycle (60+ degrees). Given the steilbel doesn’t have an internal timer do you know a way the fronius + smart meter being able to switch off heating all together (ie stop it heating all night) – I know the heat pumps are very energy efficient but I guess it’s the almost the principal that does my head in 🙂 thanks.

  19. The Off Peak/J-Tariff/Controlled Load, etc.. Is a SCAM!
    Just Get Your Sparky To Hard Wire It To The Main Supply – With Or Without Solar.

    “Thermal Mass” – Just Do The Math!

    24/7 Intermittent Operation Controlled via The Systems Thermostat Will Reduce Power Consumption Even Further Than All The Methods And Devices Stated – And That’s A Proven Fact That’s Been Field Tested For The Last 9 Months.

    Thermal Mass Requires Far More Energy To Raise Its Temperature From a Much Lower Temperature All At Once, Than To Maintain Its Set Temperature The Moment The Thermostat Senses a Drop In Temperature.

    Using Thermal Mass And Inertia/Momentum, Is An Appropriate Analogy To Comprehend The Physics Of What I’m Claiming.
    If Two Identical Diesel Powered Trucks – One Laden, and The Other Unladen, Were Travelling a Distance Of 100 Km At 100Km Per hour On A Straight Level Road –
    Which Truck Would Use More Fuel?

    • David speaks in our house 5 people usually shower at night how does this effect your idea that off peak power is a scam when the hot water system would be using most power before midnight. Could you explain as I am considering my options to save on hot water costs

  20. Peter Hanlon says

    Hi Ronald,

    Nice read.

    I have a dual element 315L hot water storage system on timed off-peak to the bottom element and normal power to the top. It is a holiday house and often goes months without use. I would like to run a WiFi controlled switch on each circuit, and control it remotely. I can see my way through most of it, but the 20 amps is a bit solid for most devices I have seen.

    Is there anything I should look at?

    I have built plenty of Arduino and Raspberry Pi stuff and will share any smarts,


    • Ronald Brakels says

      I’m afraid I can’t really help you, Peter. My idea of remote control would be to pay the kid next door to turn it on and off when I call him. But maybe some more up-to-date people will see this and help you out.

    • Hi Peter

      This is a old post, but the following may help someone thinking similarly…

      You can do this by having an electrician connect the WiFi controlled switch to an appropriately selected and sized Contactor (another name for a Relay).

      BUT why would you want to do this if you have a dual element heater?
      When you arrive at the house, turn the power on to the hot water heater and the top element will immediately start heating the water when in a peak time schedule. During the off-peak schedule, the bottom element will heat instead.

      It will take about 22 minutes to heat 50 litres, with a 4.8kw top element for water at 20 degrees to hot at 50 degrees.
      Or 30 minutes with a 3.6kW element.
      That’s enough for a 5 minute shower for one person. The next person will have to wait another 30 minutes for the water to heat.
      When the top element is operating a maximum of 50 litres will be heated if it is a ‘known brand’ heater.

      • Peter Hanlon says

        My latest notion is to use a relay such as described in the Fronius solar docs, and control it by sunny day. So I would have its ‘dull day’ with mains power linked to the top element, and after-hours off-peak linked to the bottom element. ‘sunny day’ would reverse, and I would be heating as much water as possible with my own solar production, rather than accepting the lousy feed-in tariff.I have obtained the recommended relay about $60 with delivery. The relay is a Finder, and there is a Terminal block Finder 92.03 which makes the connection pretty easy

  21. Mark calleja says

    Is a hot water timer going to save you any money if your controlled load is less than your solar feed in tarrif

    • Ronald Brakels says

      If what you pay for your controlled load is less than your solar feed-in tariff you are definitely better off keeping your controlled load.

  22. I’ve just installed a hager timer into my switchboard but not exactly sure how much i will save ,hoping you could help .
    My hot water tariff 33 is 200$ a quarter ,18hrs a day if I’m correct & have a 315 liter tank .I’ve set my timer to come on at 9am & turn off at 3pm, if don’t run out of hot water at night will soon turn it down to turn on at 10am & turn off at 2pm .My question is how much roughly will i save running at 4hrs a day instead of 18hrs ?

    • Ronald Brakels says

      It will depend on what your solar feed-in tariff is. The higher it is, the less you’ll save from taking your hot water system off tariff 33.

      If you are in the Ergon area then your feed-in tariff will only be 7.8 cents. The general tariff, tariff 11, is 26 cents. So if you took your hot water system off tariff 33 and had it switch on during the day, if half the electricity it used was solar and half from the grid the average cost of each kilowatt-hour for heating water would be 16.9 cents per kilowatt-hour. That’s less than Tariff 33 or tariff 31 so in this scenario you’d come out ahead. The larger your solar system and the smaller the heating element in your hot water system, the more solar electricity the hot water system will be likely to use. If you find you are running out of hot water then you may want to leave it switched on longer during the day.

      If you instead have a much higher feed-in tariff then your chances of coming out ahead are less.

      • My tariff 33 (18 hrs) bill is 200$ with solar feed in (7.8cents) under ergon & its been 200$ regularly & I’m knocking it down to 4hrs a day do you know how much i would reduce my tariff 33 200$ bill to? knock off 150$ a quarter maybe?

        • Ronald Brakels says

          If half your hot water system is 50% powered by solar electricity by setting it to turn on in the day, then what you effectively pay for hot water would go from $200 to around $160. But it will depend on your set up. If you have 6.6 kilowatts of solar and a hot water system with a 1,800 watt element it will use more solar energy than if you have a 3 kilowatts of solar and a hot water system with a 4,800 watt element.

  23. robin lerch says

    hi ronald
    love all your infomation. i have a hws timer installed and was wondering if i had done the right thing. i think i have
    i have a 6.6 kw system generating at the moment 32kw approx per day and a dux 250litre hws 3.6 kw heating element with hws solar fructionus timer coming on from 8am to 3pm

    what i am wondering is how do i know that my hws does not come on the tariff 11 ergon rate. i realize if its cloudy it will go to tariff 11 ie is my hws using my solar system
    at least with tariff 33 when you are connected to it, you know you never use the tariff 11. i live in bargara qld

    • Peter Hanlon says

      I have just installed a similar system. I have a Fronius Primo inverter. I am going to try a slightly different tack. Use a DC signal output from the inverter to control a Relay (DPDT) sitting in front (electrically!) of the 2-element HWS.
      So Day Sunny, the Solar power will direct to the lower HWS element, Day Dull, to the upper element. At night (same as Day Dull) , off-peak power will direct to the lower element, but hopefully just a top-up.

      The relay is about $60 delivered.

      I haven’t described it too well, but essentially change the HWS element top or bottom that you are directing power to, depending on the amount of solar you are generating.

      • Pretty sure that youre not allowed to have two power sources to the lower element if you want to qualify for offpeak power. Sure you can do it, but if you get pinged, the savings will look like peanuts compared to fine.

        • Peter Hanlon says

          I would only have one connection to either bottom or top element at any one time.

          Since this method was developed, I have taken out more performance measurements, and the off-peak tariff has moved closer to the general one.

          Also the Primo and its Datamanager allow you to configure the signal to the relay so that it has a notion of coming on to end at the end of the solar day even if the day has not reached the desirable solar level.

          So my current thinking is:-

          1. Chuck off-peak entirely.
          2. Configure data manager for turning on the DC12V signal when say 1100 watt is being generated or for 3 hours or so left in the solar day.
          3. Have the relay switch to the bottom element if signal ON, and top otherwise.
          The HWS through its thermostats will switch power OFF if temperature is reached.


  24. Peter Wallace says


    I would be very interested in an update on this subject now that the FIT in Victoria has fallen to 6.7 cents. Would a hot water timer be a viable option now?

    Peter Wallace

  25. Hi Ronald, we have just installed a 400 litre 3.6 kw electric hot water heater. We are putting 13.3kw of solar panels on a north facing roof in Sydney. Trying to work out the best way to utilise our panels and minimise our bills.
    We are hoping to use the system to run some electric floor heat in bathrooms during winter and some air con in summer. After reading about timers and diverters I am inclined to not use either.. thanks in advance

  26. Alan Champion says

    Hi – I have just installed solar in Hobart, and I think your info for here is a bit out of date.
    Currently there are two tariff options relevant to this discussion – Tariff 93 is a single meter with a Peak rate of 29.9c (7am – 10am and 4pm – 9pm), and an off-peak rate of 13.9c at other times; and a ‘flat rate’ (Tariff 31/41 which has separate meter rates for light/power (24.7c) and a lower rate heating/hot water (16.04c).
    (The old off-peak hot water tariff has been discontinued).
    So, regardless of solar, best to go for the Tariff 31/41 if you are out during the day, and tariff 93 if you are home during the day – but only if you have a timer on the hot water service.
    With solar, at a feed in rate of 6 cents, definitely essential to have a timer. If you didn’t, you could be paying 30cents for hot water if it turns on during peak times with no sun!)
    I have gone for tariff 93 as we are mostly home during the day, but have to instal my own timer at own expense – I am hoping to get one I can adjust myself as I think the best solution will differ markedly between summer and winter here.
    I have found it hard to get meaningful advice from anywhere…

  27. Malcolm Davies says

    Has anyone used the catchpower system.
    Glen Innes nsw company

  28. I think that solar timers should not be discounted. There is tech out there that looks at the weather etc and adjusts the power flow to the HWS. However, I think that the payback is too long, when people don’t spruik their products on payback, then maybe its not so good.

    We are in Qld and on ERGON, at current tariffs, the “worse off” case for us is around 20% of roof top solar use, so its a no brainer really. We are yet to look at short term night boost (on Tar 33). As we all know, its best to self consume your solar, so HWS (30% of household use), plus other appliances and change of habits will help this and get your backpack period slim.

    Cheers…James M

  29. Does this whole blog page need to be updated given that things have changed in this area.
    For example, most of us are now affected (or perhaps infected) with this thing called “Peak billing” – which in turn has reduced the price of day time electricity down a bit, (while slugging us with a fat peak billing charge between 4pm-8pm) at the same time the tariff 33 and 31 prices have increased. Also the solar feed in tariff has reduced.
    There are also more manufacturers of these timer switches, so you can get one installed for about $150 (thats $50 for the switch and $100 for the electrician who will spend no more than 15 minutes to install and wire the thing in. (speaking from experience)
    It needs to be said that it works far better for those with a 1.8kw hot water system too.
    I’ve just touched a bit on the above areas, but a fuller article would be better information for most people.

    • Jonathan Ben-tovim says

      I live in Brisbane and have just installed a timer on my controlled load tariff to power my hot water between 9.30am and 4pm. It cost me around $250 to install. Works brilliantly and have estimated will save around $125 a year so payback period is 2 years.

  30. I got a quote for a Fronius Ohmpilot – over $2400 (including installation and a 3kW heating element). So a timer looks like a bargain to me.
    Connecting the inverter to a relay in the switchboard seems like the Ohmpilot. It appears that the smarts are configured in software via the inverters’ local online server ( for me).

    • Ronald Brakels says

      You could get yourself a heat pump hot water system for that money and dramatically cut your hot water energy use that way. And some — but strangely not all — come with a timer built in.

      • Yes, my current hot water service has developed a leak so a heat pump system could be the way forward. It also appears that they work from normal power outlets. The lower power usage would keep it within the capacity of the inverter when used with other appliances – hair dryer in-particular; 5kw peak power from the solar PV/battery.

        • I replaced the old hot water service with a heat pump. It’s configured to run between 10-4pm. For all of July (to 31st) we’ve used 14 kWh of grid electricity. We have a 5kW inverter, 11kWh battery and 6.6 kW solar panels (a modest installation) located 230km east of Melbourne. This is a dramatic reduction in use of grid electricity.

  31. Ian Rogers says

    If you are on tariff 93 in Tasmania and your timer is set to heat hot water between 10 am-4pm you have the benefit of lower tariff and peak solar generation, to use as much power generated by your system as possible rather than getting the measly feed in rate.

  32. Jonathan Ben-tovim says

    This needs to be updated:
    Currently with controlled load tariffs at 19c , grid at 24c , PV export 8c,
    Payback period for a timer in Brisbane is around 2 years.

    • This needs to be urgently updated. The tariff rates quoted in this article are from a bygone era.

      From 1 July, my CL1 tariff will be up 107% from July 2022! (Endeavour network). Yep, rising from 14c to 29c.
      FiT is at most high single digit generally.

      I looked into the Catch diverter but it is not compatible with 3ph and/or battery systems.
      Their solar relay product will only work with 3kW of surplus power (which will mean for me hardly any time in the winter as the battery charging takes precedence).

      • Finn Peacock says

        We keep the info on the main website up to date – we generally don’t update blog posts (there are thousands of them going back 10 years).

        That is why blog posts have dates at the top.

        But if you mean we should write a *new* blog post on this – I agree. I’ll put it on Ronald’s ToDo list.

  33. The more I read posts on heating hot water the more confused I am getting. I am NOT an electrician. I’m in Melbourne and have a 10Kw system with a Fronius 8.2 Primo inverter with a smart meter fitted. I am thinking of disposing of my gas HWS to an electric HWS and take advantage of the Fronius data management to use surplus solar. Do I get a HWS with 1 or 2 elements? Do I need a timer? Will I run out of hot water in the mornings and run the prospect of instant divorce?
    Any help appreciated.

    • Ronald Brakels says

      Hi John

      I can definitely recommend going off gas entirely if you can manage it. Not have to pay the daily supply charge for gas is a significant saving.

      How large a hot water system you’ll need depends on both the number of people in the home and how much hot water they go through. For 3 people in Melbourne a 150 litre system is normally enough, but if you want to be sure you’ll have enough hot water you can go bigger. The smaller the element, the less power it will draw and the more likely you are to power the hot water system only with solar. A 150 litre system with a 1.8kW element will take 5 hours to go from completely cold to as hot as it will get. So if the timer is set to start at one hour before noon then on most days it will stop an hour or so past noon but the latest it will stop is three hours past noon. With a 10kW solar system there should be no trouble on most days mostly powering off solar. Even with a larger 3.6kW element you should still mostly be able to run it off solar, but avoid going larger if you can. There should be no need for a two element system provided you size your hot water system appropriately.

      If you want a hot water system that uses very little energy you can get an efficient heat pump system. But they’re not cheap and so take a long time to pay for themselves. If you’re looking to save money up front, a conventional hot water system and a timer is the way to go.

      • John Frayne says

        Hi Ronald,
        Thanks for your reply and all the info. Only 2 retired oldies in our house so we don’t use an enormous amount of hot water I don’t think. I’ve been looking at reviews and forums re HWS vs Heat Pumps and there seems to be favour in both. I’m now leaning a bit toward HP’s but can’t afford the likes of Sanden and not sure about cheaper brands. They seem to be made better now than years ago when I remember a friend having a big repair cost for one he had. What would be the most important specs to look out for if I was to go for a middle of the road cost HP?
        Thanks again for your reply.

      • Ronald,
        With the Fronius Management system operating with surplus solar heating HW will the grid come in if the weather is lousy all day?
        My peak is 3pm-9pm.
        Can you fit a timer as well to heat HW say early in the morning? Is this feasible?

  34. So just to make sure I’ve got this right. I live on the NSW Northern Rivers. I have a 6.2 Kw solar system split 50% each facing due north & west. My hws is 259lt with a 3.6kw element and whilst in excellent condition, is 15 years old. We are a retired couple home during the day.

    Switching it from CL to solar either with a timer or diverter is going to save me so little that I won’t even break even on the cost of the transition before my hws says yibbada yibbada, that’s all folks and packs it in from old age.

    So I’m guessing I’m best off leaving things as they are and investing in a good heat pump unit when the day arrives?

    • Anthony Bennett says

      Could well be Mark.

      Diverters are a bit expensive but a Catchpower Green can have both solar and CL fed into them.

      Timers and contactors are cheap enough. If you have lots of solar available then standard HWS are a simple and reliable solution. Get someone to throw a sacrificial anode and a 1.8 or 2.4kW element into it and it’ll be more likely to run for free during the day.

      Heat pumps are super efficient but difficult to fix if they go pear shaped.

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