Hot Water From Solar Panels: Three Ways to Do It

A "solar panels hot water system" can mean two different things, so it is worth settling which one you mean first. Solar hot water uses roof collectors to heat water directly: the Australian Government's Your Home guide describes the water being heated by the sun as it passes through the collectors and flows into an insulated storage tank. Solar panels, also called solar PV, make electricity instead.

This guide is about the second meaning: using the electricity from rooftop PV to heat water. Your Home says electric hot water systems can be powered from the grid or from rooftop solar panels, and that includes both standard tanks with an element and heat pump water heaters.

So a home with PV does not need collectors to get sun-heated water. It needs a hot water system that runs on electricity, set up to do most of its heating while the panels are producing. If you are weighing up roof collectors instead, the solar hot water guide explains how those work.

Running hot water off solar panels comes down to timing. The Australian Government's Solar Consumer Guide on energy.gov.au says the best way to save money is to use more of the electricity your solar system generates and less from the grid, and gives typical windows for that: about 10am to 4pm in summer and 11am to 2pm in winter. It also suggests checking your solar monitoring app for the hours that suit your own system.

So an electric tank or heat pump that does its heating in that window uses your own generation for as much of the job as the panels can cover. The catch is the size of the load. The same guide says that if an electric hot water system uses more power than the panels are generating, it imports the difference from the grid, and that this is more likely with a large traditional element tank than with a heat pump, which uses less electricity. That is why the three ways below differ so much in how much of the heating your panels really cover.

A timer is the simplest way to shift an electric tank's heating into daylight. Your Home says most heat pumps and some newer electric storage tanks have built-in timers that can be set to run during the day, when excess solar is most likely, and many electric hot water systems let you set those timers from a phone app. If there isn't enough solar in the timer window, the system uses grid electricity instead, so you still get hot water on a dull day.

An older tank without a built-in timer can be fitted with an external one. Your Home warns that the timer window must not be set so short that it overrides the tank's built-in protection against Legionella bacteria. Fitting an external timer is electrical work for a licensed electrician.

Smart controls go a step further. Your Home describes a home energy management system that watches the excess solar and switches the hot water on at the right time, keeping grid use down while making sure the tank is fully heated. Some smart electric tanks also have variable elements that match their power draw to the surplus.

A PV diverter, also called a solar diverter, sends surplus solar into an electric tank's element. Your Home explains that a diverter monitors the excess solar electricity and throttles the power supply to the hot water system to match it, unlike a timer, which switches the tank on for a set window.

The same Australian Government guide is frank about the downsides. It says solar diverters are relatively expensive, are not well covered by Australian standards, can cause electrical disturbance, and may void the warranty of your hot water system. It adds that they may override the tank's built-in protection against Legionella bacteria, and that they are not compatible with heat pumps.

So a diverter is only an option for an element tank, and it is worth reading the tank's warranty terms before one goes in. Installing a diverter is electrical work for a licensed electrician, and the question to ask is whether a built-in timer or smart control would do the same job with fewer of those risks.

A heat pump on a daytime timer is usually the easiest load for rooftop solar to cover, because it needs much less electricity to heat the same water. Your Home puts a heat pump's coefficient of performance, the heat it delivers for each unit of electricity, at around 3 to 5, against about 0.95 for a new electric storage tank. A smaller draw means the panels can carry more of it, and less is imported when the sun is weak.

Your Home says many newer heat pumps include app monitoring and timer control so they can use excess power from a home solar system, and that a heat pump's energy use can also be reduced by timing its heating for when the air is warmer.

Your Home also notes that a heat pump powered by solar PV offers many of the same advantages as a solar hot water system and can sometimes be cheaper to install and run, and that PV can power other appliances too, which may make it a better use of roof space. Installing a heat pump in Queensland needs a hot water installer with a QBCC solar and heat pump endorsement.

Moving hot water off controlled load and onto your own solar can save money, but not in every case. energy.gov.au says shifting a controlled-load hot water system to run on solar can be a good idea, but not always, because the costs and benefits depend on your electricity pricing plan. Controlled-load power is already billed at a lower off-peak rate than the rest of the home, according to Your Home, and those circuits are increasingly being switched on during the day to soak up the grid's surplus solar.

Your Home's view is that if you have solar or a time-of-use tariff, a timer or smart control may suit you better than controlled load. Some heat pumps can't run on either controlled load, and others only on Controlled Load 2 (Tariff 33 in Queensland), so the model's manual matters too.

The other side of the sum is what your exported solar earns. In South East Queensland, the Queensland Government says retailers set their own feed-in rates and those rates differ between retailers. Your current rate is on your bill, and it is the number to weigh against what you pay for grid power before deciding whether to heat water with your solar or export it.

You can work out most of the picture without touching anything. These are safe owner checks:

1. Find your feed-in tariff on your electricity bill, and check whether your hot water is billed on an off-peak tariff.

2. Open your solar monitoring app and note when your system generates most on a typical day.

3. Look in the hot water system's manual or app for a built-in timer, and for which tariffs the model suits.

4. Compare retail offers on the Australian Energy Regulator's Energy Made Easy website, which the Queensland Government points to for feed-in rates.

Everything past that is licensed work. energy.gov.au says converting hot water from controlled load to solar needs an accredited electrician, and Queensland's Electrical Safety Office says to always get a licensed electrician for any electrical work, including connecting hard-wired appliances. Timers at the switchboard, diverters and rewiring a circuit are all jobs for an electrician. A new heat pump needs a hot water installer with a QBCC solar and heat pump endorsement.