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

This article 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 SolarHotWaterQuotes: “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.

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)Save

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.