Your biggest load is heating while you sleep.
Electric hot water is often the single largest item on an Australian bill — and it is usually wired to run at 1am, where solar can never reach it.
What hot water actually costs
A household getting through about 150 litres of hot water a day uses roughly 8.64 kWh doing it — around $1,009 a year at 32 c/kWh, a typical New South Wales residential rate. That is more than the fridge, the dishwasher, the television and every standby load in the house combined.
150 L a day through a 40 degree rise is 6.97 kWh of heat, plus about 1.5 kWh of standing loss from the tank.
We show the arithmetic because the published figures disagree wildly — guides commonly quote 2 to 4 kWh a day, which is not enough to heat that much water no matter whose system you buy. Physics is a better source than a comparison site.
The controlled load trap
Most electric storage systems in Australia are wired to a controlled load — a separate circuit on a discounted tariff that heats the tank overnight. For a house without solar this is a sensible arrangement and it saves real money.
For a house with solar it is close to the worst possible arrangement. The largest single load you own has been deliberately scheduled for the hours your roof produces nothing, while the power it could have used is exported for a few cents. The discount is real — but it is a discount on electricity you are still buying, and no controlled-load rate in Australia is as cheap as using your own generation.
A year at the general rate. A controlled-load tariff lands somewhere below this and well above the figure beside it.
The same energy, costed at the 5 c/kWh you gave up by not exporting it.
Three ways out, and what each one really involves
1. Re-time it to the middle of the day
The cheapest change, and the one with a genuine catch. Moving off controlled load means giving up that discounted tariff, so any kilowatt-hour your solar does not cover is bought at the full general rate instead. On a sunny day you win comfortably; across a wet week you can lose. It suits a system with some headroom, and it needs your retailer and a licensed electrician to change the metering.
2. Replace the element with a heat pump
A heat pump moves heat from the air into the water rather than making it with an element, doing the same job for about a third of the energy — 2.79 kWh a day instead of 8.64. Just as usefully it draws under a kilowatt rather than 3.6kW, which a modest solar system can cover while a resistive element often cannot. Federal and state incentives usually apply.
3. Send surplus solar to the tank
A diverter watches what you are about to export and puts it into the hot water element instead. It keeps the tank on its own circuit and only uses genuine surplus, which avoids the tariff gamble in option one. It is extra hardware, so it earns its place on a system that exports a lot rather than one that barely covers the house.
A tank of hot water is stored energy you have already paid to put there, and it holds it well enough to carry you through to the next morning. That makes it the cheapest storage most homes own — not a replacement for a battery, since it will not run your lights, but a very good use for power you would otherwise sell for 5 cents.
Work out your own
Often on an overnight controlled-load tariff — heating 150L through 40°C rise plus tank standing losses (~8.64 kWh/day).
It runs on its own schedule rather than yours — there are no hours to set. The figure already allows for it switching itself on and off through the day.
Most of these run all year. Drop it if you only use this one seasonally.
Most published running costs quote one national rate of about 33 c/kWh. Yours is on your bill, and it is the number that decides everything here.
Moving this into daylight means giving up the discounted overnight tariff it is probably wired to, so the saving depends on your solar actually covering it. Our hot water page works through the catch.
Estimate only. Running on solar is costed at your feed-in rate — the earnings you give up by using the power instead of exporting it, not zero. Appliance figures verified 2026-10-03; default rates 2026-06-22.
Frequently asked questions
How much does an electric hot water system cost to run?
For a household using around 150 litres of hot water a day, roughly 8.5 kWh a day — about $2.72 a day or just under $1,000 a year at 32 c/kWh. Published guides often quote 2 to 4 kWh a day, which cannot be right for a family: heating 150 litres through a 40 degree rise takes 6.97 kWh of heat before you count any standing loss from the tank. If you are on a discounted overnight tariff your bill will be lower than that, but the energy used is the same.
What is controlled load, and why does it matter for solar?
Controlled load is a separate circuit that heats your water on a discounted overnight tariff, typically in the small hours. It is a genuinely good deal if you have no solar. If you do have solar, it is the trap on this page: your single largest load has been deliberately scheduled for the one time of day your roof produces nothing. The discount is real, but it is a discount on power you are still buying.
Should I move my hot water to run during the day?
If you have solar and enough of it, usually yes — but it is a real decision rather than an obvious one. Moving to a daytime timer means giving up the controlled-load tariff, so every kilowatt-hour your solar does not cover is then bought at the full general rate. On a sunny day you win comfortably. Across a wet week you can lose. It suits a household with a system sized with some headroom, and it needs your retailer and a licensed electrician to change the wiring and metering.
Is a heat pump hot water system worth it?
It is the strongest version of the same idea. A heat pump moves heat from the air into the water instead of making heat with an element, so it does the same job for roughly a third of the energy — about 2.8 kWh a day instead of 8.5. It also draws under a kilowatt rather than 3.6kW, which means a modest solar system can cover it comfortably while a resistive element often cannot. Federal and state incentives usually apply.
Is hot water really a kind of battery?
In the useful sense, yes. A tank of hot water is stored energy you have already paid to put there, and it holds it well enough to carry you from afternoon to the next morning. Heating water with surplus solar is one of the cheapest forms of storage available, because the tank is already on the wall. It does not power your lights, so it is not a substitute for a battery — but as a way of using solar you would otherwise export for a few cents, it is hard to beat.
Which is the bigger saving: solar, or a heat pump?
They solve different halves of the problem, and together they compound. A heat pump cuts the energy needed by about two thirds. Solar cuts the price of whatever energy is left from 32 cents to roughly 5. Doing one gets you most of the way; doing both takes the largest load in many homes down to a rounding error on the bill.
Get your free solar quote in under 2 minutes.
Tell us a little about your place and we’ll tailor the numbers. Everything in the list below is included, no exceptions.