How To Divert Your Excess Solar PV to a Hot Water Cylinder.

a diversion sign

Is it worth investing in a system that will divert your excess solar electricity away from the grid and into your hot water system?

This guest blog post has been written by experienced solar installer, electrician, solar company owner and all round good guy, Mark Cavanagh from MC Electrical in Brisbane.

Mark answers a really common question we are getting here at SolarQuotes: “Can I divert my excess solar energy into my hot water cylinder instead of selling it to the grid for a pittance?”

Take it away Mark:

With the reduction of the solar feed in tariff across Australia, and battery storage not economically viable yet, there has been an ever-increasing interest in developing ways to use the electricity you produce rather than feeding it back to the grid for 8c per kWh.

An obvious solution presents itself: divert the solar electricity into your hot water system. But before we jump into it, let’s do the maths, working with typical peak and off peak tariffs available in Queensland.

Assume we have a 5kW solar system and a 2.4kW element in our hot water system. Assume your hot-water usage is reasonably high and you are using around 10kWh a day on Queensland’s Controlled Supply Economy Tariff 33 (T33).  T33 is cheaper than your regular tariff because it is only guaranteed to be available for 18 hours per day.


Most hot water systems are able to comfortably run off Tariff 33. The above system would use 10kWh day x 365days = 3650 kW/year. At the T33 rate of 20.3 cents your hot water would cost $759/year. If however, you ran all your hot water off your excess solar power (worth 8c if you exported it), you could save 12.3c/kWh or $448/ year.

Two methods of using solar electricity to produce hot water


Method One. Install a timer.

Install a load shift timer. Set the timer to run when your solar is running. If you set it to run it from 10am till 4pm, your water will usually heat up in the middle 4 hours of the day, and use the power that your solar would most likely have sent back.

Cheap. Installed for around $220.
Reliable. Just one timer with battery backup in case you lose grid power.
Effective. If used correctly, your hot water will heat off solar most of the time.
Simple. It’s simply adjustable by the user (should your lifestyle change).

Dumb. If you turn the kettle and toaster on at Saturday lunch-time, your home will draw more power than your solar is producing, then at 3pm when the solar is still operating, it will turn off as it will have heated from the grid at 28c/kWh. If used in the wrong situation, a timer could end up increasing your power bill.

Assuming at best you could heat 70 per cent of your hot water with your timer – (the remainder would be on the standard domestic Tariff 11 at 27.9c/kWh). (3650x .7 x.08)+(3650x.3x.279)=204+305=$509 Compared to $759, this is a saving of $250

You should, by these figures, pay for the installation of your time switch within 1 year, and go on saving $250 a year.

Method Two. Use a solar diverter.

immersun controller

The “ImmerSun” hot water diverter from RFI Solar. Yours for about $1k installed.

A “Solar Diverter” is a clever little device that tells the solar electrons to go to the hot water instead of feeding back to the grid. The benefit of this method is that it’s totally interactive with solar production and the household load. As the demand from the house and the production from the solar varies, the diverter will automatically adjust so that solar production will be prioritised: first to the house, second to the hot water, and at last resort, to the grid. As an additional benefit, (providing you have a tempering valve on your hot water) you can turn your hot water system right up from 60 degrees to 80 or 90 degrees, giving you increased energy storage capacity. There are two products on the market that seem to be leading the way, the immerSUN and the SunnyMate. Are they worth it?

The immersSUN is currently distributed by RFI. RFI Solar have been in the renewable energies game for over 30 years so they should be around to back the 3-year product warranty. An obvious disadvantage of the immerSUN is that it is limited to 3kW -larger hot water systems often have 3.6kW elements. They are selling for $924. Or let’s say, $1100 installed. Australian Wind and Solar recently stopped supplying the immerSUN because they were having too many faulty units (they have sent me photos of the old stock because I was suss at this claim!) They have now introduced their new product the SunnyMate.

apollo gem (sunny mate)

The SunnyMate is a rebranded Apollo Gem controller from the UK.

The SunnyMate is rebranded Apollo Gem device (UK).  The three-year-old company Australian Wind and Solar are importing it, and are offering a 5-year warranty. I have installed one as a trial at my parents house, and I must say it is impressive box of electronics. Watching it react to and only send 100w to a 3.6kw element is mystifying!  It was installed in July 2014 and it seems to doing its job without a glitch (as of January 2015). The SunnyMate is badged at a maximum of 3.5kW, but AWS say that it will still be covered under warranty if used with a 3.6kW element. They are taking pre-orders until April for $550 + postage but claim the price will go up to $880 + postage + installation. Let’s say it will cost $1100 installed.

Smart. You will not unnecessarily feed your solar back to the grid for a measly 8c.
Flexible. You can super heat your water, so it stays hotter for longer.
More suitable for non-routine households, where they are not out of the home just about every day, but still are feeding in 10kWh to the grid.
Will be ideal if the feed-in tariff is reduced further. (some feed in tariffs in Qld are down to 6c).

It’s expensive. (Approx. $1100 installed)
It’s new – and not yet proven in the Australian market. (however I have personally trialed it without a flaw now for 6 months as of Jan 2016).

Without a bypass switch, your  hot water will run 100 percent off solar, so it will be 100 percent savings (3650x 12.3) = 448 a year.
(AWS conservatively claim a four person family can save nearly $400 a year.) You should, by these figures, pay for the device in 2.5 years and go on saving $448 a year.

However, if you use less hot water than 10kWh a day, your payback time will increase proportionally.

Efficient Alternatives

If $1100 is hard to swallow, there are other alternatives. If you have a 250 litres or larger hot water system you may want to consider putting it on Tariff 31 (cheaper at 12.9c because it is only guaranteed to be available for 8 hours per day). Your local sparkie should be able to sort this out for about $150.

Update Jan 2016 – This part of the blog used to recommend heat pumps. Installing hot water systems isn’t my field of expertise, but after talking with many plumbers and customers about their unreliability, I have stopped recommending heat pumps as an economical solution. I’ve explained a new and economical way of diverting solar power in this blog.


Installing a load shift timer is a simple and cost-effective way to reduce your power bill assuming you generally do not use a lot of power between 10am and 2pm. As a smarter option is the SunnyMate, while the outlay is considerable – you can see a return for your investment in under 3 years.

There are other efficient alternatives. It may be worth looking into installing your hot water on Tariff 31. While I no longer recommend my customers to lash out and install a new heat pump, if you have a bit of cash and are looking at installing a new inverter, considering using the smarts of the Fronius.

About Mark Cavanagh

Mark is the owner and manager of Brisbane based solar installation company MC Electrical. He is an electrician and an accredited solar installer with over 20 years’ experience.


  1. I would love to see these figures for Western Australia. Only one electricity supplier in Perth area. No T33 rate.

    • Mark Cavanagh says

      Hi dj, I can try to crunch the numbers if you like.
      What do you pay per kwh? 24.6c ?
      Do you have a feed in tariff?
      And how much hot water would you use? (EG: That of an average 4 person family, or?)

      • Thanks Mark. I have power supply that has a price variation with time of day.

        43 cents on peak mid day week day
        11.5 cents off peak morning and evening week day
        21.9 cents high shoulder day time week end
        18 cents low shoulder night.
        that is sort of the basis. see smart power from Synergy for the detail. It puts lowest rate at night and highest during high use periods during week days. the high rate changes (swaps) times from summer and winter.

        I live alone so manage my power quite well but always looking for better ways. I also use a hot water system that is a heat pump. Not solar but quite cheap to run. I need to sort out the best time to run it but that is a challenge as I need to plot better running conditions vs rate for power.

        • Mark Cavanagh says

          Hi DJ, We don’t have “Time of Use” in Queensland, unfortunately we’re really behind on that – but ill give you my 2 cents worth.
          First a question. Do you have solar PV? If so, what size and do you get paid if you send solar back to the grid?

          Second: You already have a economical heat-pump – so forget the ImmerSun or Sunnymate!- Being 1 person with a heat pump you would use very little power to heat your hot water! However you could consider putting a timer on your heat pump and running it at your 11.5cent times. I’m not sure if this is common practice in W.A. There may be regulations or reasons (running conditions) not to do this – sorry I cant help there. Call your local sparkie for advice!

          • Hi Mark thanks for the reply. You have confirmed what I expected. Not much advantage to try to support the hot water system. I do have a 3kw solar system. It is not the best as I am forced by size to install on NNE or SSW. I chose NNE for the install and do see a substantial return to the grid. As I use most of my power during evening. Have looked at installing more panels on the west side but so far that is not really an economical solution. (YET) I will wait and watch.
            I get paid 7.135000 cents per unit. I return about 427 units and use 892 units. That is why I was looking for some place to use or store the units I return. I have looked at an option to make better use of the units I have but cannot really tell if it will make that much difference to the price.

            I’ll just wait and watch. It will get better. Keep up the good work. I read your and other news on alternative power.

  2. Andrew schadow says

    Hi there just Q could you divert excess power to run your air conditioning

    • Mark Cavanagh says

      The short answer is no Andrew. It only works with resistive loads ( eg: heating elements).
      The longer answer is yes with the use of the multifunction relay, but it wouldn’t be interactive, so it wouldn’t be worth doing.

      • Tom Martin says

        I really don’t need to add heat to my solar thermal hot water system during the solar day when I have maximum heat generation and minimal use. My HW is already super heated by the end of the solar day without a boost from my PV system. Can the Fronius inverter’s Appliance Relay or PV Diverter run a compressor or heat pump that chills a tank of antifreeze containing a heat exchanger? This super-chilled tank could be used in reserve after the solar day to help my AC cool my home.
        I also asked this question on your Fronius blog page but this may reach a different audience.

    • Andrew McAlister says

      i am currently replacing my 2kw pv system (with sunnyboy 3000tl inverter, no battery) with a 6.5kw pv system with battery, gridconnected. i will be able to leave the current system if disconnected from grid. My solar/gas boiler is 10yr old, and i am considering using tbe old pv system to heat water. Do you think i could run that inverter disconnected, and with one output socket to run the boiler. Pv system produces 1.5kw (winter) to 10kw (summer). i think there is enough power, but i dont know if that inverter can run off grid, or if it can support a socket directly. what happens if the water gets hot and switches off, but the inverter is still producing enery that is not used. is it a problem if electricity is not removed from the inverter?

  3. Hi Mark, Great article and good to see someone taking the time to provide valuable information to the public. Top work.
    Just a few key notes-
    1 New Immersuns just don’t work, I know because I have 40 of them and cant even give back to the manufacture.
    2 I would check the comment about RFI selling Immersuns, don’t think that’s the case.
    3 SunnyMate name clash with “major manufacture” not the case and resolved. good rumour that got traction and attention from the major players but resolve.
    4 SunnyMate / GEM deal – made a few engineers at AWS cry but that’s not a fair call. Working on a product for 16 months with the help of other manufactures isn’t the same as slapping a sticker on it. again another competitor rumour.
    5 I would never recommend adding a time clock to save costs as a comparison to a SunnyMate. Its not comparable.

    Great team at AWS head office are always welcoming calls to carry out research. We have a SunnyMate on the way for you to try out.

    Keep up the Great work up north.

    • Mark Cavanagh says

      Thanks for the info Adam, would be keen to try one out!
      1) Interesting, hence my comments to Jake about the SolarImmersion.
      2) I just called, and RFI are apparently selling them – is there something we should know about that?
      3) Thanks for clearing that up. Was a nice rumour though.
      4) Apologies- I retract that comment for what it’s worth!
      5) Where do you think I went wrong? Assumptions for my calculations?

      • You neglected any loss of feed in tariff in your timer calculation. In many cases the use of a timer will result in a negative effect.

        so 3 hours with a 3.6kw element results in 10.8kwh. But in Melbourne the average peak output of a 5kW solar system is 2.8 kW So .8kW will be imported from the grid per day on average. This means that in Victoria the use of a timer and a 3.6kW element (average size in OZ) will result in a larger cost than simply leaving the hot water on off peak. Here’s the maths.

        OFF peak only cost.

        10.8kWH *.15c/kWh =$1.26 (But you have to account for the loss of feed in tariff I’ll use .06c so it actually works out to $1.26-(10.8*.06=.54)=.72c per day for hot water.

        Timer used form 11 till 2

        the load is 3*3.6kW=10.8kWh

        But the average peak output of a nth facing 30 degree 5kw solar system in MElb is 2.8kW, So the 2.4kW remainder is imported from the grid at peak rate ( I’m using .25c/kWh)

        So 2.4kWh*.25=.24c EXTRA PEAK CHARGE.

        So the use of a timer costs the loss of feed in tariff + the extra Peak costs which will be .54c+.24c=.78c/day for hot water.

        OP only = .72 But if you install a timer it costs .79, so no matter how small the capital expenditure, it will never pay for it’s self, even in Queensland it will only result in a very small savings and most likely take much longer than 3 years to pay back. The maths say that the SunnyMate comes out on top.

  4. David Porter says

    I see your reviews of both solar pv diverters. It’s a shame all the work done by the original inventer Robin was taken by 4eco.Ltd.
    Little do you know that a law suit is brewing in the UK over his invention and alot of installers are dropping the immersun like a hot stone.
    I would be more cautious claiming who makes the sunnymate just because it looks like the Gem.
    Facts are stronger than just claims

    • Mark Cavanagh says

      Fair call David, I’ll admit that was just what I understood from talking to AWS and others. Read Adam’s comments on this blog.

  5. Hi
    I heard about another solar PV switching device solarimmersion – which is made in UK as well. It is quite popular across the country and in some parts of Australia as well. There are a lot of threads running in different forums about this device. Wonder if anyone knows about this?

    • Mark Cavanagh says

      Hi Jake, I looked into this briefly before blogging, and on the surface it looks good – but when i contacted SolarImmersion they did not have a supplier in Australia. For me that takes it out of the equation, as I never import products because of the risk to me and my customer. My take – if it’s a has any major advantages, a reputable retailer will cotton on and bring it in soon enough!

  6. Fordstanden says

    We have 2 x 250 liter tanks side by side -the first as a low wattage element connected to the solar system heating only in the middle of the day controlled by a timer. Water then flows to the second tank to be topped up at night rate ( if required ) giving a huge reduction in our power bill.
    The same could be done with a tank with both an upper and lower element.

  7. Wendy McCarthy says

    Hi Mark, We have an electric hot water service that heats up at night. We have solar panels and I was just wondering if we pushed the boost button during the day for the hot water service, would it then be cheaper to heat up at night because it would already be partially heated.

    Thank you


  8. Hi Mark,

    nice blog good work!

    Can you please let me know one or two recommended makes/models of HWHP on the market that would be suitable for a house with 2 to 3 adults living? (ie a smaller unit, pref under $2K)

    I will run it on a timer during middle of the day as I have 2.5kW solar PV system already installed. house is in Margaret River WA

    many thanks! Benny

    • Michael F. Camille says

      Hello Mark,

      Been looking at reducing my electricity cost for hot water for a while now. Interested to see if the heat pumps system would be beneficial to me.
      I have a 2kW solar PV system.
      Hot water system is a Saxon ThruFlow 140ltrs with a 1.8kw element. Hot water used mainly for showering morning and evenings. Solar buy back is around 550kw (Quarterly) at 6cents.

      Thanks for your informative blog.


  9. Hi Mark,

    Have you seen the Fronius inverters with the Datamanager built-in? These have an energy management function built-in also which can then switche on a load depending on how much surplus energy there is. You should check it out.


  10. Nice blog, very informative, heaps thanks

  11. As suggested above, some Fronius inverters can make existing electric hot water service heating a relatively simple addition to a PV setup. I have a Fronius Symo 10 with its wifi connected Datamanager and Smartmeter. With the addition of a relay on the HWS’s circuit and selecting a few on/off choices on the inverter’s Energy Manager webpage, our big power-hungry HWS is now being re-heated after everyone has had a morning shower on sunny days, rather than only at night. At worst, when the sun doesn’t shine, it automatically drops back to the existing overnight off-peak mains supply. I could also use the off-peak meter’s manual boost button in desperation should everyone decide to take consecutive baths on a dull day.

    The result being when overnight off-peak kicks in, the HWS is already near optimum temperature and uses very little power to recover. The on-line setup means I can adjust the heater’s Feed-in/Consumption on/off wattages remotely on any device/browser if weather or household demands require it.

    • Wendy McCarthy says


      that is a great idea but do you have to be careful what else you have on (electricity wise)


      • Quite true, however if you set up the on/off trigger wattages conservatively, it’s unlikely to incur any excess consumption. I have it coming on while feed-in to the grid is over 3000W and it goes off when the demand from the grid is in excess of 500W.

        Since the HWS consumes about 3 – 4kW, when the solar output is just over 3kW, the HWS will click off almost immediately due to its own consumption tripping the 500W limit from the grid. So the HWS is really only on when there’s a fairly decent excess from solar generation, and with a small risk of using 200 – 300W from the grid, on top of what may be draining normally during the day.

        There’s plenty of leeway in the Fronius settings to cover the vagaries of any setup/location. The Energy Manager also has Time-On and Daily Maximum, so you can set it to stay on for x minutes after being triggered by a gap in the clouds, and then set an overall limit to x hours per day if you wish, although the HWS thermostat would kick in once optimum water temp is reached. After only a few weeks since my PV install, and frequent wintry weather, I’m surprised how often it’s managed to do a daytime boost all the way to tripping its thermostat off – hot water fully reheated by the sun.

  12. Thomas Morgan says

    The conclusions that it draws are incorrect. The use of a timer can in many cases increase the cost to heat hot water. Timer switches only make sense with very small elements (2.5kW or less) and excessive PV arrays. >5kW. Anything less and the customer will find themselves in a negative cash position. The average element in Australia is 3.6kW with many areas using 4.8. The use of a timer under these conditions will cost the customer much more than if the hot water system was left on off peak rate

  13. Carioca da Gema says

    Hi Mark!

    I wonder if a set-up could be arranged so that all the output of the PV panels goes through a battery + inverter system such as the one from Bosch and any surplus above battery charging goes straight into a small inverter-powered heat pump such as the Daikin mono or the Daikin A/C Ururu Sarara?

    In other words, never export anything?


    LMH a.k.a. carioca

  14. If you can get a heat pump for $2k and installed for that, I would run a mile. It would have to be crap with a COP of say 2. A good quality Sanden is far more expensive with a COP of around 4, but still won’t do the job with excess PV a lot of the time. And the cost in excess of $3500 installed. For an extra $1k you can have a Evac Tube solar hot water system installed and pay virtually nothing for hot water per year. I pay less than $20 most years.

    Finn and Mark get your facts right on all matters including the kettle BS example!

  15. Donald Pickwick says

    Hi to all you Solar gurus,
    Wow, what a mind boggling adventure looking into this forum. Wish I had seen this before I purchased my 6Kw system from Euro Solar. Maybe someone can steer me straight on my lack of understanding;
    1. prior to purchasing our 6Kw system our quarterly bills sat around the $800 mark
    2. post installation and 12 months down the track our usage has remained the same yet we are only seeing a reduction in our bill of $100 per quarter
    Now that I have read your tariff costing’s above I may need to review these or should I call the service department of Euro Solar to come check the system?
    or am I expecting to much from the system?

  16. Phil McKinnon says

    We’ve relocated from Sydney to Ballina in Northern NSW, and construction of our home should commence in the new year. Obviously using solar for our power and HWS was a topic to be explored, but we have had no prior experience in using either. In looking at what we could find on the web, I soon realised this whole was a bit of a minefield for me, not so much the logic as to why I should not pursue an installation but the posts by folks on product reviews and the like that relayed horror stories on problems and performance issues with even those products that Finn rates so highly. Given the initial costs involved it is something which you would want to get right.
    So after taking Finn’s feedback we have spoken to providers up this way and decided on a Solax 5kw inverter and Seraphim panels. The recommended solar HWS system was a split Envirosun solution, but on checking on the web that does not seem to get a lot of positive press in reviews; nor for that matter do a lot of the other brands available. So my quandary is why go with Solar HWS at all, just get an electrical HWS and run that off the Solax system using a timer to sort of replicate the off-peak reheat scenario. This would reduce the HWS install considerably. There’s only the 2 of us, so the water is only used for showering and any hand washing up; the dishwasher and washing machine will be using cold water. So presumably running costs would be minimal. Is there any downside to my logic?

    • Phil, true there is no reason to go with a $1100 solution.
      How much is your KWH rate for hot water ?
      I live in Maitland and mine is 8 cents a KWH ,hardly worth doing anything to change as the cost difference is so small.

      • Hi Eric,
        Apologies for the delay in responding, but we’ve only been in the house for some 5 weeks now. We signed up with Click Energy, primarily because my brother and niece who live nearby are with them and haven’t had any issues with them. We will be on their suggested plan which we can review with them after 3 months once we get some usage and feed-back data. This plan gives us 10c per KWH feedback. The cost of the timer which diverts the solar power to the HWS was $140. The timer runs from 8am – 11am and we will monitor and or change that as we settle in. So far so good and we have not had any problems in having nice hot showers etc. Also we were informed by the builder that for NSW you can’t install an electric HWS unless you do what we’ve done; otherwise it has to be gas or solar. We don’t have any other gas in the place so that’s another reason why we opted to go down the path we have. Hope this helps.
        Cheers, Phil.

  17. The figures for NSW are different.
    The cost of Electric hot water on a 84716 meter Controlled load 1 is 8 cents not anywhere near the quoted high tariffs.
    That equates on a 30 day period for an average house hold to around $20 to $24 a month.
    Considering feedback is 5 cents the saving would be 3 cents which is about $4 to $9 dollars a month or $72.00 a year.
    Unless these figures change the cost of the unit to divert is not a viable solution

    As a matter of fact i just wonder why people would pay these high tariffs??
    i made them remove the smart time of use meters a few years ago and just have a 24229 meter at a rate of 22 cents any time of day and the 84716 meter at 8 cents so I am puzzled as to why people are paying high tariffs .
    There was a time when i was told once the Smart meter was installed and operating it could not be change back because the government would not allow it.the after a few years it all changed and I changed companies and asked to go back to standard metering and they came out and changed it all so that is why i can’t understand what is going on with some of these users.

  18. Just adding my experience with the Sunnymate, which was installed with our 4kW PV system last June. I have recently collated a whole lot of data from the mains meter of our house and combined it with the data available for the gross PV generation (provided with the PV system) and sequential readings from the Sunnymate device itself. We are a 4 person household in the Hunter region of NSW. We use the Sunnymate to divert excess PV-generated energy into our standard 315L electric HWS.
    The results are encouraging, we have had no problems other than the fact that the PV installer and the electrician were both unfamiliar with the Sunnymate device and it took a bit of fiddling around to get it right. Otherwise, easy to use and to set timing etc.
    We went from a gas HWS to the electric one and instead of paying around $6000 for evacuated tubes, we opted for a $2500 standard one and most days we are getting full hot water heating from the sun, in addition to the energy we use elsewhere in the house during daylight hours. During this colder weather, we are using over 80%of the energy we are generating, and as our FIT is only 5.1c/kWh, we are happy with that.

    • Finn Peacock says

      Thanks Suzie, that’s really useful.

      Is your electric hot water heater a heat-pump?

      How much did you pay for the Sunnymate + its installation over and above the Solar install?

      Best Regards,


  19. Chris Allen says

    What a great blog. Western Australia August 2016. Gas has leaking gas so needs to be changed. 6kw solar panels 3kw east and west … Two 2.5 kW inverters .. Average 1kw each side across the day… With approx 2.5 AV anytime. 4 plus kW available say 10 am to 2. Use say 55 percent of power with pool spa dishwasher etc all spread through the day. Thinking of switching to electric hws with one of those sunny mate controllers mark spoke about at top of blog., wa feed-in tariff 7c … Winter with pool spa etc only have 2 hrs of excess solar to play with. Any wise advice ??

  20. Hi
    Are you sure this value (3650 kW/year) is correct?

    There is huge different between kWh and kW-Y

    • it looks right to me I use 10KW and over per day for 3 people and I use the washing machine and the dishwasher to heat their own water and we aren’t shower mad sometimes i will go a couple of days without taking a shower .The other day i switched off the hot water and switched it back on at 6PM next morning I took a reading and the hot water had used 12.3 KW I guess it pays to keep the water hot.

  21. Thanks for the info it has made my mind up to not buy a heat pump system and to stay on the grid at 9 cents a KWH as I use 10KW a day I will just wait till batteries come down in price. I use 8 to 9\kw of solar ,10KW of hot water and 8to10KW of mains power so my system might just make the cut as it can produce up to 35KW a day.

  22. Would it be wrong to connect 6 X 250 watt panels directly to an electric element without using a thermostat and just letting any excess energy boil off in a similar way that traditional solar thermal hot water heaters do?

    I am trying to avoid switching damage in the thermostat that would be done by a dc current

    • Ronald Brakels says

      My first impulse is to say don’t do it. While the circumstances are assuredly different, there is similarity enough for my memory to hark back to this passage from Paddle Wheels and Pistols:

      “Many engineers not only made steam with the water as low as possible, bu they tied the safety valve shut so that it could not blow off and relieve the pressure. Thus bottled, the boiler was compelled to stand stresses for which it was never designed. If luck were with the engineer his engines ran better than they knew how, but if luck were bad the rivets surrendered, the tubes gave way, and in the sudden expansion of the steam the unfortunate boat was rent from end to end. Men were blown through solid wooden bulkheads, were lifted from pilot houses and thrown a hundred feet, and lived unhurt, like the survivors from a freak hurricane, but most of those on board a steam boat whose boiler burst died at once, or were horribly scalded or crushed.”

      • Looking back ,my real question should have been, are the over pressure valves on the solar thermal hot water heaters any different to the over pressure valves on electric hot water heaters?

        • Ronald Brakels says

          I’m afraid I don’t know. I’m not qualified to give advice about the workings of hot water systems and that’s why my previous comment wasn’t very helpful. But if I suspect something may be dangerous, my default position will always be – don’t do it. However, someone else who knows more than me might be able to help you.

    • Finn Peacock says

      Hi Brendon,

      That is a very interesting question. I’m not an expert on the Australian Standards for water heaters and there could be something in there that requires a thermal cut off with an electric element.

      In theory, as long as you have a failsafe safety cut off – so the thing can’t boil dry if the water supply stops it should be OK. But I would have a good read of the standard.

      Hope That Helps,


  23. Ronald and Finn

    Thank you for your replies.
    I think I may have to look at some solid state switching via the thermostat, unless somebody else can explain if it is OK to boil off the excess energy in the same manner that the solar thermals do.

  24. Hi folk
    I am helping (I hope) a friend who is building a new house for her retirement here in Brisbane. She had said she wanted both pv and solar hot water systems as well as air con. One supplier has recommended she instals a few more panels and a solar diverter. Pl can you advise what size solar system she might need to have a hot shower on a hot day in summer. If she runs air con all day will there be any current left for hot water

    • Ronald Brakels says

      Hello John. Installing solar PV and using it to power an electric hot water system can be cheaper than installing a solar hot water system. But because diverters are still fairly expensive it can be cheaper to put the hot water system on a timer so it turns on during the day when solar power is being produced and use the money saved to install extra panels.

      Because a retired person or couple generally don’t use a great deal of electricity, if she installed a 3 kilowatt inverter with up to 4 kilowatts of panels she should have more than enough solar electricity for hot water whether she installs a diverter or puts her hot water system on a timer. If she wants to be certain of having enough solar electricity, or wants to help out the environment by producing extra solar electricity or in the future wants to install batteries or get an electric car, without getting special permission she can install up to a 5 kilowatt inverter and up to 6.66 kilowatts of panels.

  25. I’m having a PV system installed and this is an issue I’m currently looking into. So far the timer switch is the only easy part solution but it has the downside that it may even cost more to do it that way if not enough solar power is being generated (i.e. cloudy day) then it will need to import from the grid so some days may end up being an expensive solution. One solution for this would be to detect the solar power level being generated that would determine when the heater is switched on. If insufficient then heat overnight on off-peak. The problem with this it requires a solar installation large enough to drive the heater element 100% so no import required.

    The issue here is that the heating unit is “dumb”. If the heater power required is more than what the solar is capable of producing we are importing again. One solution would be to heat the water over a longer period of time by reducing the element current. It probably requires a phase detection circuit and only switch on for part of the mains phase (equally on both sides of the cycle) which should reduce the power. Ideally a control unit would also want to detect the water temperature so that it can be set to the desired temperature and also control whether it needs to switch on during off-peak hours to top up if not heated enough during the day.

    I don’t whether any of the units mention in this topic employ that method of control. The manufacturers are probably not likely to tell you how they actually work.

    • I am trying to make sense of all this Stewart and am hoping some of the collective wisdom on this website might be able to come up with a simple and cheap solution for me. I have spend weeks tweaking the system and think I might have that one right, so now need to look at hot water heating which is still uncertain.

      We currently have a solar heater (Sydney) which requires no external heating in summer but does require a boost in winter. I am adding a further 8 kW to the existing (old) 2 kW PV system so I suspect I will have a bit of energy to play with. What I want to do is control WHEN the booster heats as well as do this with a low rate of heating over several hours.

      I would like to incorporate a timer into the system to turn on booster heating (via a relay???) for around 4 hours in winter kicking in around 11 am and for around 2 hours in summer kicking in around 3 pm.

      Anybody in the room able to suggest the best way to do this in 2017? It seems possible but not a lot on this sort of tweaking.

      • Ronald Brakels says

        Hi Mick.

        You could use a diverter so your hot water will only boost using solar electricity under normal circumstances, but the expense may not be worthwhile if it is only required in winter. Using a timer is a currently cheaper option, and with 10 kilowatts of solar panels, even on cloudy days much of the power is likely to come from your solar panels rather than the grid. One thing you could do is set the timer so it starts later than 11am to give sunlight more time to heat the water before it gets boosted.

        Another option is to do nothing or put the booster on off-peak electricity. Given that feed-in tariffs in NSW can now be over 12 cents a kilowatt-hour and putting a hot water boost on a timer or a diverter probably won’t reduce your electricity bills by a great deal, this isn’t an unreasonable choice.

        • Hi Ron.

          Thanks for the suggestions.
          I read about the diverters and these sound great. The cost makes them unviable at this stage though.

          To give you more information about my system I omitted to mention that we also have an Apricus rooftop solar hot water heater in addition to the 10 kW PV system.
          Today I had the sparkie put the booster on a timer and we set it for 2 hours a day. My only concern is that the water heating element is a 3.6 kW element and this may result in sucking all of the power generated by the panels and put them into water heating at the expense of household usage.
          One option is of course to find a 1.8 kW element but not sure if this is available or if there are other issues I may not have thought of.

          Oh yes…I will be jumping off the ‘Time of Day’ tariffs and onto the ‘Anytime’ tariff tomorrow as this should smooth out any potential cross-over with the HWS.

          Any thoughts on this? Will find out if this works in the next month or so.


          • Ronald Brakels says

            Hi Michael

            If the sun is shining, with your 10 kilowatt PV system you should be okay with the hot water system drawing 3.6 kilowatts in the middle of the day. If it’s cloudy then it will no doubt use grid power. Let’s hope you are not in Melbourne. Clouds and cold weather go together and you’ll use the most hot water when it’s cold, but even so, with your Apricus system I’m guessing you don’t have to do a lot of boosting. You could get a smaller element and boost it for a longer time, but I expect this would save you less than $100 a year. Possibly much less.

            I am certain that with a 10 kilowatt system you would be better off with an “anytime” standard tariff than a time-of-use one, so I expect that will save you a little bit of money with that change.

          • Thanks Ronald. You’re probably right.
            I tried to get the ‘Anytime’ tariff but found out the hard way that Ausgrid removed this tariff a month ago.

  26. Hi Mark,
    i have currently 5kws hybrid inverter (sungrow) with lithium battery with 18 panels and my problem is i have electric hot water,i was thinking I’m going to get solar hot water system with electric booster,just wondering with electric booster can possible use my pv panels to boost if hot water runs on off peak at the moment(flexible rate tariff i have been told by distributor)i used to have dedicated circuit but now is change.whats the best options (timer?)honestly i need some ideas or opinion?thank you

    • Ronald Brakels says

      Hello Gina, Ronald here. You can put a timer on either your current electric hot water system or on a solar thermal hot water system with electric boost. Because a solar hot water system will use very little electricity it may not be worth the expense getting a diverter installed. Unfortunately, both solar hot water and solar PV will perform poorly during prolong cloudy periods, so a timer will only reduce and not eliminate grid electricity use for hot water.

      As you have a battery another option would be to switch to a time of use tariff and use off-peak electricity to heat water if required.

  27. Might be time to revisit some of the opinons/advice given here. Sandens, Siddons and (some) other heat pump hot water systems do seem to be getting a good rap from some quarters as quiet, reliable and efficient. According to several Tasmanian acquaintances saved “heaps”.

    Catchpower diverter is another option now available in the diverter stakes.

    Interesting to note that Victoria has increased FIT to 11 cents from July 2016. I pay $0.0755 cents per kWh for controlled load to momentum, So I’m actually better off feeding to grid. I could get a slightly lower peak price but overall I’ve calculated that i’m better off paying a slightly higher peak price and much lower controlled load price. It remains to be seen what will happen in 2018

  28. I’m seeing ads on the net for “Powerdiverter” which positions itself in this space but I haven’t seen it mentioned.

    • Ronald Brakels says

      Hi Neil. I’ll be writing about “powerdiverters”, also called “hot water solar diverters”, soon.

  29. Is Catch Power also in that space?

    • Ronald Brakels says

      Hello Neil.

      Catch Power is a diverter that will use surplus solar electricity to heat water (and potentially for other tasks). I am going to write an article on it very soon.

  30. Peter Childs says

    Gday Team. I was reading the Install Manual on the Bosch Compress 3000 HeatPump and it appears to have a ‘open’ or ‘closed’ contact for Solar PV with some options to basically run the pump when closed/or open from the Solar Inverter.

    I don’t know anything about inverters and/or heat pumps but I though this was interesting rather than doing strange things with invertors/divertors/relays/etc.

  31. Hi guys
    I have decided to install 2 elements to our home made s/s 450 litre gravity fed water heater the original 3.6 kw element will be moved to a socket about 1/2 way up ( formerly used for anode which after 15 years is as new) and will remain grid connect to off peak power with thermostat set at 40 °c . The original element socket will have a 2.4kw element with its own thermostat set on 75°c and will be powered through dump relay on redback inverter which is a 5.2 kw offgrid back up system.

    • Ronald Brakels says

      Hi Ash. I am not a plumber, but my understanding is the tank at set at 40 degrees will be required to periodically be heated to at least 60 degrees to prevent legionella and other microorganism growth.

      • Bob Johnson says

        Hi Ron (the followig is all for SE Queensland),
        You are correct- the thermostat must be set at a minimum of 60degrees to prevent the growth of Legionella bacteria and a tempering valve fitted to restrict the hot water tap temperature to 50degrees. Interestingly, these are for new instillations so there are many old solar hw systems with a manual boost switch (I have several friends with this set up) where the temperature could possibly vary from quite cool to literally boiling hot (kids beware!).
        On the topic of HW systems- I thought a lot about this 2 years ago when building a new house (solar or heat pump just too capital expensive and gas prices going only one way) and ended up going the simplest way. 315L system on Tarrif 31 (17.36c/KWhr inc GST). With the retailer giving 14c/Kwhr FIT I figured the money I would have saved if I fed the solar to the HW would probably be more than used up when it had to be topped up with the standard Tarrif 11 (26.96c/KWhr). This works very well with very low power bills on a 5KW PV system and a very variable load (kids visiting home, occassional Airbnb in the granny flat etc). However, next time perhaps a twin element system and a more expensive inverter with the dedicated out put to the lower element etc…………..hmmmm.

  32. I’m in WA on the early 2011 47c incentivised FiT. 3Kw inverter with 3kw panels facing NW and 2Kw facing NE. There is curtailed (clipped) power in middle of the day in summer when both PV banks are putting full output and can be in excess of 4.3Kw at times. Only 3Kw will ever be produced by the inverter and I’m wondering if there is a controller that can push the excess energy ie up to ~1.3Kw into the water heater. Its gas but I can install an element in it. This would be a good way to utilise spilled energy in the form of hot water.

  33. Hi,

    I want to replace my old gas hot water heater with a electric hot water heater and add a control load meter to my meter box. Is it possible when I install solar panels I can include a diverter to heat the hot water with my excess solar energy and is this option more economical than the control load price of 11.660 c/kwh incl.gst? My husband and I are retired so we are at home during the day.


    • Ronald Brakels says


      If your solar feed-in tariff is higher than the controlled load price then you are better off putting your hot water system on the controlled load and getting the solar feed-in tariff for your solar electricity. Even if your solar feed-in tariff is only 8 cents you might be better off with the controlled load as diverters are still kind of expensive. (But if your solar feed-in tariff is only 8 cents I’d suggest checking to see if you’d be better off with a different electricity plan.)

  34. I have a question for all the diverter gurus. Just had a 5.4 kw system installed. All good apart from a wayward roof screw touching the neutral in a previously poorly positioned cable. Part of the deal was to install a diverter for the HWS. However the HWS was instantaneous LPG so the plan was to install a 400 L storage tank and run a designated line from the distribution board to the new HWS. The actual meter is on a shed 50 metres away. There is no J or off peak tariff meter nor a designated line for a HWS from the meter to the house. The solar PV supplier supplied a blue catchpower diverter. Everything is 3 phase including the inverter. So the bottom line is the catchpower can’t be fitted because my understanding is that it has 3 clamp on cable sensors that have to go on the mains side of the meter and 3 load sensors that have to go on the load side of the meter. So unless I install one single wire 50 metres underground from the meter to the HWS it aint going to work. This blows the economics completely out of the door. The supplier has agreed to take the catchpower back . I like the idea of the diverter, there is only 2 of us, so we went over the top with the size of the HWS to make the thing whole work as close to 100% solar as we could. I had 3 quotes and only the eventual chosen one had any idea of what we were trying to achieve. However I am still slightly annoyed that I have now had to spend hours researching this because the people I dealt with couldn’t get their heads around the issue. It is not that rare to have a meter separated from a distribution point. Any suggestions appreciated.

  35. Hey Mark, I have a question about hot water on a designated circuit heating over night. Can you please tell me if there is a way to change how long the hot water is heated for. I can hear it dripping on the roof when if has heated enough which if usually around midnight. Then I can hear it switch off at 8am in the morning. Is there someway to shorten the heating time on the smart meter? Thanks, Karen
    Grampians Historic Tobacco Kiln

  36. mark krawczuk says

    ya got that wrong, : At the T33 rate of 20.3 cents your hot water would cost $759/year.$ 759 a year to run hot water , more like that figure in one or 2 quarters,…

  37. Ive done the same for a total cost of about $75 give or take. I installed a smart switch on my water heater (wifi connected). I bought a raspberryPI and configured HASS (Home Assistant on it) and connected it up to read the energy readings from my Solar inverter as well as the connected power meter (both wifi connected). It is then programmed to turn on the water heater anytime that the solar power produced is greater than the max draw on the water heater (1600 watts) and the power being sent to the grid is greater than 1600 watts. Then a separate step to turn the water heater back off as soon as I start drawing power from the grid again.

    Not quite as precise as being able to send exactly 100 watts to the heating element but it works for 1/5 the cost. Maybe I will look into how I can do that.

    • Great stuff Nathan. I like the concept, just not sure in my case how easy it would be to pick up inverter output and the meter output. I have looked into something similar and this is what I have come up with. I have purchased a 10W solar panel, a 5W halogen globe for a load on the panel. I then got an adjustable low voltage disconnect module to connect to the 10W panel and a DC to 240V AC relay to feed a 1200W AC element in the HWS. I intend to adjust the voltage cutout to match when the roof solar panels are producing in excess of 1200W. As you say not as efficient as the diverters, but in my case we had a 3 phase solar setup with remote power meter and it all became to hard to get a diverter that would deal with all 3 phases for any sort of reasonable price. As a pure indulgence I am also looking at a stand alone small wind generator with its own element as a further backup. We live in a semi rural area on the coast with plenty of wind.

  38. Xavier Robinson says

    Solar PV

    I have 2 fronius 5kw inverters and a smart meter 13.2 kw panels and export limited to 5 kw I make up to 80kw a day but can really only export up to 40 kw in perfect conditions .
    Where to next I have
    *rinnai b 26 70c model can i use any pv to boost
    *a mid size pool with 2 pumps and inverter heater , small amount of passive heating on roof
    * wanting to put in hydronic heating

    Live in Ocean Grove 3226

    What should i do next to use my surplus and maximize savings ???

    • Ronald Brakels says

      Hi Xavier

      I think a rinnai b26 is a gas hot water system. In case I am mistaken and it is electric you can put it on a timer so it switches on during the day when your solar system will be producing power. If your home uses a lot of hot water you can look into getting a hot water diverter.

      Your pool pumps and heater can also be set to run during the day to take advantage of solar electricity. As it’s likely to provide the most power in the middle of the day that’s when you want whatever draws the most power running. But rather than run everything at once you may want to stagger things through the day. For example, run a pool pump a few hours after the sun rises and then have your hot water system switch on closer to the middle of the day when there there should be more solar output and so on.

  39. Mark

    Not sure if this been covered, but seriously I get lost easy on this subject.

    I’m basically want to design a system for my mobile home. Solar panels to battery storage and then excess for hot water. Hot water use will be minimal 3 minute hot shower and few dishes a day.

  40. An update to this article on latest technology would be most useful. Thanks!

  41. I have an instant gas. Can I preheat with a shunt and an electric. It am I better off to just go electric. ? We do export a lot and still have a lot of clipping as we gave a 12 kw solar with a 5 Kew export limit. We are in 3226 VIC. And on 11c export and 20c flat tarif. Thx Xavier

    • Ronald Brakels says

      Because of the risk of legionella it’s not permitted to preheat water in a tank and then send it to a gas instant heater. It would be possible to use an electric instant heater that uses excess solar and warms the water before it goes to the instant gas heater, but that won’t be very practical. It would be easier to just switch to an electric hot water system. To get the hot water system to use mostly solar energy there are a number of options:

      1. Put it on a timer so it switches on in the middle of the day.
      2. Use a relay that switches it on when there is enough surplus solar power.
      3. Install a hot water diverter that will send small amounts of surplus solar power to the hot water system.

      Going off gas altogether can be financially worthwhile because it saves you having to pay the daily gas supply charge. But whether or not that is worth it to you will depend on your preferences.

    • Geoff Miell says

      You state: “I have an instant gas.” and: “We are in 3226 VIC.”

      I’d suggest you read Tim Forcey’s article headlined “Goodbye to Bass Strait gas”, dated 15 Apr 2020. It includes:

      “Here’s something you might not have heard. In eastern Australia in 2025—just five years from now—we have no idea where up to half of our wintertime gas will come from. If you haven’t heard about this, it’s because few people are talking about it.”

  42. Sean McHugh says

    I have a 2-element hot water system with only the off-peak element connected.

    Online I have seen fairly inexpensive 300W, 12V solar panels. I have also seen inexpensive 1200W, 36V elements. How would it go having 3 of those panels in series (36V) going to a 1200W, 36V 2nd element? I’m assuming there would be a thermostat in the tank which I could ensure is inline.

  43. I had good results by putting my electric hot water on a timer to use my generated solar power. Changed the element to 1800w so as not to exceed capacity of solar (5kw) on cloudy days. It got rid of the big off peak component from night heating of water. Then took up government offer to change to a heat pump for free and my bill went higher. Heat pump has now gone on a timer.

    • Ronald Brakels says

      I had heard the heat pump systems that are cheap enough to be covered by the STCs received for installing a heat pump hot water system were cheaply made, but I didn’t realize they were so cheaply made as to not include a timer. That’s a bit rotten.

  44. If the unit has a facility to put it on a timer it is not obvious. No instructions included beyond basics. I used a “Bunnings” timer to drive a 12v power supply that operates a relay installed in the line to the thermostat. Works from 10-00 to 12-00 then 15-00 to 17-30 to give a final heat up before sunset. Thought this would be less intrusive to the unit than powering it on and off via a time switch twice a day.

  45. I’m just an interested bystander curious about the subject and wonder about a setup that doesn’t seem to be mentioned anywhere here…probably for good reason, but lets see what you think.
    Say you had an electric powered reservoir that was not connected to the grid at all and was just powered by rooftop solar. I assume you could set the thermostat so that it would stop drawing rooftop power once heated…and perhaps wind up the temperature at which it stops heating…?
    Say you connected this up to an on demand gas heater that would then only have to kick in when the feed water from the reservoir was too cold.
    Obviously the demand for hot water would affect the size of each unit, and I imagine there would be different situations where the effectiveness would vary.
    My questions…
    1. Would the on demand gas hot water systems work reliably with variable temperature feed water?
    2. Is this actually a sensible alternative (considering the start-up cost of two different water heaters).
    3. Is there a break-even point where this would be effective for individual home owners, for larger apartment buildings, for industry, or anywhere at all?

    • David
      On demand (instantaneous) gas hot water systems are used as boosters for solar thermal hot water systems, which is little different to what you are suggesting.
      1/ Modern instantaneous gas units have controls which adjust their output according to the temperature of the incoming water to maintain a constant output temp. The ones used with solar thermal systems have some slightly different components that can deal with the potentially very hot incoming water, but they are available
      2/ An electric water heater is just an insulated storage tank with an element in it. They are fairly cheap so adding an instantaneous gas unit wouldn’t make the system crazy expensive. Certainly cheaper than a solar thermal system.
      3/ Too many variables & outside my skill set to guess at break even points

      • Thanks Peter. Good to know you don’t think it’s crazy. Yes, I was a bit ambitious with question 3…but sometimes it’s amazing the kind of stuff some people can tell you.
        Thanks again.

  46. Hi Mark
    Its a bit old, but thanks for the blog. I think that your savings for HWS on solar should be based on the full tariff is 100% used.

    We are looking at 10kW roof top and aim to use as much as we can…central Qld. One suggestion made to us re HWS was to change the element to say 1.8kW for a lower load and longer time. We average 8kWhr/day (down to 6kWhr in summer). We could increase the thermostat setting to boost the temperature to ensure gets past the night. We are getting new appliances eg washing machine etc but these have simple timers – max 9hrs this will not be smart run.

    I am thinking of a timer for the HWS and we monitor from there.

    As you know the energy production curve from roof top solar varies during the day, so there will always be some export/import unless one goes to batteries (which have a longer payback period)


  47. Hi Mark,

    I enjoyed your article and it is exactly what I am trying to weigh up – I would like to use my HWS as my second battery.

    Does anyone have a more up-to-date list of smart diverters and any reviews/feedback?

    Thanks in advance,


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