The short answer
It depends on how long the room will be empty. For an hour or two the difference is small, so choose on comfort; for a day or longer, switching off or turning the setting back usually means less heat for the system to move.
On this page
You'll hear two opposite pieces of advice: that switching on and off wastes energy, and that nothing should run in an empty room. Each is right in some situations. Knowing what actually uses the energy makes it easier to decide for your own home.
What decides it
In summer, heat flows into your home from outside; in winter, it flows out. Either way, it flows faster when the gap between the indoor and outdoor temperature is bigger.
- While the system is off, the room drifts towards the outdoor temperature. As the gap narrows, heat flows in or out more slowly, so over a long absence there's less heat for the system to deal with overall.
- When it comes back on, the system has to remove the heat that built up, or replace the heat that escaped, including heat that soaked into or out of walls, floors and furniture. It does that at high output, where most systems are less efficient than when they're ticking over. What an inverter does explains why.
- In cold, damp weather, warming a cold room back up also means more defrost pauses and a slower recovery.
The longer the room would be empty, the more the first point tends to outweigh the others. Over a short break, the difference is small either way.
Switching on doesn't use a burst of electricity in itself. Most wall-mounted systems use an inverter, which starts the compressor gently and builds up speed. The energy goes on moving the heat that built up, or leaked away, while the system was off.
A rule of thumb for each situation
| Time away | Cooling | Heating |
|---|---|---|
| An hour or two | Leave it running | Leave it running |
| Out for the day | Off, or a few degrees warmer | Off, or a few degrees cooler |
| Overnight | Sleep mode or an off timer | Turned down, not off, when it's cold |
| Several days | Off | Off, or frost protection if available |
Turning the setting back rather than off is a middle way. The room doesn't drift as far, and it recovers more quickly. A timer or app can bring the system back to your normal setting before you return; see timers and schedules.
When leaving it on makes sense
- Rooms that heat up fast and hold the heat, such as loft rooms and rooms facing the afternoon sun. In a heatwave, once they've soaked up heat, the system may take hours to bring them back down.
- Humid weather. With cooling off, moisture builds up again and the room feels clammy when you come back.
- Cold, damp spells in winter, when warming a cold room takes a long time and involves defrost pauses.
- Short gaps. Switching off for a few minutes achieves little. Most units also wait a few minutes before restarting the compressor after it stops, to protect it.
When switching off makes sense
- Rooms you won't use again that day.
- Mild weather, when the room won't drift far from a comfortable temperature anyway.
- When a window needs to be open. Cooling or heating with a window open sends much of the system's work straight outside.
- Going away. Switch off at the remote. Before turning the power off at the isolator for a long period, check the manual, as some models need the power back on for several hours before they're used again.
Test it in your own home
The answer depends on your home, your weather and your habits, so your own meter is the most reliable guide. Some apps estimate how much electricity each indoor unit uses, and a smart meter's in-home display shows your whole home's use.
Try one approach for a few days, then the other, in similar weather, and compare the kilowatt-hours (kWh). To turn a difference into money, multiply it by your unit rate. For example, a difference of 4 kWh at an example rate of 25p per kWh (check your own tariff) is £1.
On a time-of-use tariff, when you use electricity matters as well as how much, because the same kWh costs different amounts at different times; see time-of-use tariffs. For working out running costs more generally, see how much does air conditioning cost to run?.
Related guides
- Timers and schedulesMost systems have on and off timers that count down a set number of hours, and many controllers and apps add clock times and weekly schedules. Used well, they have a room comfortable when you need it and the system off when you don't.
- How much does air conditioning cost to run?Multiply the electricity it uses, in kWh, by your unit rate. How much a unit uses depends on the room, the weather, the temperature you set and its efficiency, so estimate it from its average power draw and the hours it runs.
- What an inverter doesAn inverter lets the compressor change speed. Instead of switching on and off, the system runs hard to reach the setting, then slows down to match what the room needs, holding a steadier temperature with less noise.
- Time-of-use tariffsTime-of-use tariffs charge different unit rates at different times of day. Whether one suits you depends on when your air conditioning and the rest of your home use electricity, and how those rates compare with what you pay now.