Sunshine On Tap: Turn Your Hot Water System Into A Battery

Don’t you hate it when you’re out and realise you’ve left the gas on at home? It’s a nightmare when we do it with a gas stove, yet millions of Australians are essentially doing this every day to keep their water piping hot. There’s a more efficient alternative: use a hot water heat pump as a thermal battery.

Gas Hot Water Systems Are Money-Burning Machines

All over the country, steam rises from water kept piping hot by the lick of blue flames. Gas hot water systems are so extraordinarily wasteful — especially the storage varieties that keep water continuously hot for whenever it is needed — that they are the major reason why hot water accounts for a fifth of Australian residential emissions.

These systems aren’t just burning gas: they’re burning money. Phasing them out would provide combined annual savings to Australian households of $4.7–6.7 billion by 2040, by which point gas households will pay $660–960 per year more than those with a heat pump water heater, according to University of Technology Sydney research.

The future of hot water is electric — and especially for homes with a solar system — a hell of a lot cheaper. But what good is solar when you need to do that last load of dishes after dinner or an evening shower before heading to bed?

Fill Up The Tank With Cheap Solar

The answer is simple: use your hot water as a battery.

Owners of electric hot water systems can configure them to heat the water when the sun is out to take advantage of abundant solar energy: all that hot water is stored, insulated inside a tank, ready for when you need it, serving as a thermal battery.

This is also great for the grid, transferring electricity use from periods of high demand to times when energy is plentiful, and providing voltage reduction that minimises the need for rooftop solar curtailment.

A recent University of New South Wales (UNSW) study found the average Australian household could save up to $400 a year on electricity by installing heat pump hot water, and powering it in the middle of the day during peak solar generation periods.

The study, based on a four-person working family with an average daily hot water consumption of 200 litres, found gas is financially competitive with electric hot water today only if you excluded the daily supply charge, which can cost up to $360 a year in Australia even if you don’t use a single puff of gas.

Switching to electric hot water is one of the key steps to disconnecting from gas entirely, removing that expense once and for all.

The UNSW study found the lowest annual water heating bill was provided by a household with rooftop PV and a premium model heat pump set to work with a timer during solar soak hours.

Resistance Is Futile (For Some)

The research also indicated that if a resistive electric hot water system has reached the end of its life and needs replacement, there are benefits to upgrading to a heat pump, which is more efficient and delivers 67% less emissions on average.

Payback period cases for replacing a resistive system with heat pumps fall within warranty periods — which is key given how short many heat pump warranties are.

Resistive electric hot water systems are cheaper to purchase but use about triple the energy of a heat pump.

SolarQuotes in-house installer Anthony Bennett says that both options can be effectively used as thermal batteries:

“A typical 300 litre hot water unit stores approximately 14 kWh of energy when heated to 60°C from an ambient temperature of 20°C. A simple resistive element tank costs about one ninth of what a home storage battery does, if you don’t already own one, so even if it “leaks” 1 or 2kWh/day in standing losses, it’s still worthwhile. And if we look at heat pumps using 25% of the electricity consumed by an element while still delivering the same amount of hot water, then you may still have enough power to charge your EV and heat your home in mid-winter.”

Your hot water system installer should set the timer to heat your water from solar energy and/or off-peak electricity – make sure they show you how to change this yourself after they have finished the install.

More Than One Way To Heat A Tank

There are a number of different approaches to ensuring your hot water system is making the most of cheap solar energy. UNSW project lead Dr Baran Yildiz tells SolarQuotes he recommends using a heat pump’s inbuilt timer (check our comparison table to confirm which systems have one and what type if so), or buying an external timer.

Yildiz says:

“The heat-pump’s internal control (if available) would be preferred over the timer control as it won’t interrupt the operation and cause any potential impact on its compressor unit and lifecycle.”

Setting the timer for 11am through to mid afternoon gives the best chance of coinciding with peak solar generation. External timers can also work with resistive electric hot water systems.

If you are having the system installed, Yildiz suggests making sure the installer takes care of the timer for you:

“Some installers by default offer to install timers for the electric water heating systems when they install solar systems and program the timers to run during solar period. Getting this installation organised while the installer is already on-site might save users a secondary call-out fee.”

You can go further by configuring your solar inverter to only activate the hot water system when there’s actually enough solar available. Anthony says you can set up your system to “turn on a load when your solar has ramped up to a certain yield … solar diversion is the common terminology, because you take electricity destined for export to the grid and divert it into your own storage or consumption.”

Another option is smart switching, which Anthony says “can be used for both resistive and motor loads (like heat pumps and pool filters) but may need to be programmed for the appropriate solar threshold and time window.”

“You want to switch on when there’s enough sunshine to run the load but not on and off every other minute. For this example the diverter activates once there’s 4kW of export (and stays ON for 25 minutes) if the sunshine wanes to 2.2kW after that time, then it switches off,” he adds.

For owners of resistive hot water systems, dynamic solar storage can squeeze the most value out, particularly when all parts are working within the same manufacturer ecosystem. “With one app everything is visible and control is at your fingertips,” Anthony says.

It isn’t always as efficient, but the simplest option of all is having your heat pump connected to the controlled load with the right retailer, which Yildiz recommends particularly for people without their own solar:

 “These days most DNSPs expanded controlled load circuits to daytimes.¹ The key here is to find a retailer which offers cheaper rates during solar-soak window so they benefit from excess solar generation in the network.”

Goodbye Gas Anxiety

For more on heat pump hot water systems, read our comprehensive guide. Once you’ve gotten quotes for your preferred system, you can look forward to disconnecting from gas, and rid yourself once and for all from that nagging sense that you’ve left the gas on at home.

Correction: The original version of this article wrongly stated that the emissions from hot water exceed that of transport, at a fifth of Australia’s carbon emissions. This is incorrect: hot water represents a fifth of residential emissions, not total emissions, and does not exceed that of transport. 

Original Source: https://www.solarquotes.com.au/blog/sunshine-on-tap-turn-your-hot-water-system-into-a-battery/