The 'leave it on' idea
The argument for leaving the air conditioner on is that it takes a lot of energy to cool a hot house from scratch. That is partly true: the first stretch after switching on uses the most power. But a house left at 24°C on a 38°C day is gaining heat through the walls, roof and windows the whole time, and the air conditioner has to remove all of it. An empty house allowed to warm up gains heat more slowly as it gets closer to the outside temperature.
On a flat rate, the energy used to cool the house down when you get home is usually less than the energy used to hold it cool all day. Modern inverter units also ramp up and down efficiently, which makes switching on and off less wasteful than it once was.
When leaving it on, or pre-cooling, can save
| When | Rate | Cost of an hour |
|---|---|---|
| Solar soak, 11am to 4pm | 17.59c | 26c |
| Off-peak, 9pm to 11am | 22.6c | 34c |
| Peak, 4pm to 9pm | 47.64c | 71c |
- Time of use: cooling the house in the cheaper afternoon window, then easing off in the peak, can cost less than starting from hot at 5pm.
- Free midday power or solar: on the Solar Sharer Offer or with rooftop solar, running the unit in the middle of the day costs little or nothing.
- A well-insulated home: holds coolness for longer, so pre-cooling carries further into the evening.
- Health: if anyone at home is vulnerable in heat, keep it on. Comfort and safety come first.
Running the air conditioner smartly
Use the timer
Start cooling 30 minutes before you get home.
Shade before you leave
Closing blinds on north and west windows cuts the heat the house gains all day.
Set 24°C to 27°C
The SA Government's recommended cooling range.
Close off unused rooms
Cool only the space you are using.
Your own numbers
A medium split system costs about 50c an hour on the Ausgrid reference rate. Our air conditioner calculator prices your own unit and hours.
Questions about leaving the air con on
On a flat rate, usually not. A house kept cool all day keeps gaining heat that the unit has to remove. Switching off and using a timer before you get home generally uses less energy.
On a time-of-use plan, with free midday power or with rooftop solar, cooling in cheap daytime hours and easing off in the evening peak can cost less. It also makes sense when someone at home is vulnerable to heat.
Starting up from hot does draw more power at first, but modern inverter units ramp efficiently. Over a day, holding an empty house cool still usually uses more.
Sources
Every rate on this page comes from the regulator or the regulated retailer. Last checked October 2026. Prices include GST unless we say otherwise.