There’s a piece of conventional wisdom that gets repeated every winter: don’t turn your heater off overnight, because it takes more energy to heat a cold house back up than it does to just keep it ticking over at a lower temperature. It sounds reasonable. It’s also wrong, at least in my own home.
I ran a two-week test comparing the two approaches head-to-head. The result: turning the heater off completely overnight used noticeably less energy 12.7% less energy than lowering it to a setback temperature.
Here’s how I tested it, what I found, and why it happened.
My test
I ran two back-to-back one-week trials in my own home, using the same heater and the same target outcome each time – a target temperature of 19°C by 7am. The only thing that changed was how the heater behaved overnight.
Week 1 – Lower the temperature overnight I turned the heater back to 16°C overnight, then manually turned it up at 6:30am with a target of 19°C by 7am.
Week 2 – Turn the heater off overnight I turned the heater off completely overnight, then set it to start automatically at 6am, aiming for 19°C by 7am.
The heater settings (target temperature, fan speed, zones) were identical between the two weeks. The heater automatically turned off at 9:30pm each night. I manually checked indoor temperatures during both trials to confirm the house was reaching a similar temperature by 7am each morning, so the comparison was measuring energy use for an equivalent result rather than comparing a warm house to a cold one.
The weeks were back-to-back, so weather was consistent but not exactly the same. I logged into my Powercor My Energy account and downloaded half an hour interval data to measure our energy usage between 10:00pm and 7:00am the next day. Apart from the fridge and a few lights, the only difference should have been the heater.
While the test was only over two weeks and not exactly done in laboratory conditions, the settings, routine, and target result were consistent across both weeks, which is what makes the comparison meaningful.
What I found
Lowering the heater to 16°C overnight and boosting it back up in the morning used 12.7% more energy over the week than switching the heater off entirely and letting it start fresh at 6am.
That’s a meaningful gap for what looks, on paper, like the more “sensible” option – many of us assume a lower, steady temperature must be more efficient than a big swing from off to on. The data says otherwise.
The average low temperature for Week 1 was 9.86 degrees. The average low temperature for Week 2 was 8.10 degrees, which strengthens the result further. Turning the heater off and back on was more efficient despite the heater having to work harder work in Week 2.
Why turning it off wins
The explanation comes down to heat loss, not heating effort. A house loses heat to the outside at a rate that depends on the difference between the indoor and outdoor temperature – the bigger that gap, the faster heat escapes through walls, windows, and gaps in the building envelope.
When the heater is set back to 16°C overnight rather than switched off, the house sits at a higher average temperature for several more hours than it would if the heater were simply off. That higher average temperature means a bigger indoor-outdoor gap for longer, and more total heat leaking out over the course of the night – even though the heater itself is doing less work at any given moment. Reheating a fully cooled house from ambient temperature in the morning takes a burst of energy, but it turns out that burst is smaller than the extra energy spent keeping the house 3 degrees warmer for eight-plus hours.
Some might respond saying that this might not be the case in a more thermally efficient home – but I disagree. The principles are the same. Our home is not thermally efficient and might drop to 12 or so degrees overnight. If our house was thermally efficient and say only drops to 15 degrees overnight, turning the heater off will still save more energy as less heat will be lost. The home will require less energy to heat it up in the morning than keeping it warm overnight. I would expect the difference to be less, but the principles are the same.
The takeaway
If your priority is minimising energy use, switching the heater off rather than running it at a reduced temperature is worth trying in your own home. In this test it meant using 12.7% less energy, which in a time of rising energy bills, translates to meaningful $$ savings. And if you do turn the heater off, it’s best to set the heater to come back on automatically before you wake up. It’s nice to wake up to a toasty warm house!
If you are interested in a quote for electric heating and cooling or a heat pump hot water unit, we’d love to hear from you.
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