With an amber heat-health alert in force and the mercury pushing towards 35C in parts of the country, the air con is barely getting a rest. And every heatwave brings back the same tempting belief: that cooling a hot room back down from scratch costs more than simply holding it cool, so you may as well leave the unit running all day, even when the house is empty. It is one of the most widespread and most expensive myths about cooling a home.
The myth of leaving it on 24/7
The logic sounds sensible. Switch the unit off and the room heats up again, so when you get home the compressor has to grind away for ages to pull the temperature back down. Better to avoid that effort and keep everything steady. The flaw is that this ignores what happens during all the hours the house sits empty.
A cooled room does not hold onto the cool air. It constantly leaks heat back in through walls, windows and doors, and the bigger the gap with the outside temperature, the faster that cool escapes. To replace what leaks away, the unit keeps working for hours on rooms with nobody in them to feel the benefit.
What running it empty actually costs
The figures show the scale of the waste. A roughly 1 kW unit, at an electricity rate of about 26p per kWh under the Ofgem price cap for 1 July to 30 September 2026, costs around 26p an hour to run. Left on around the clock that is more than 6 pounds a day, and close to 188 pounds over a month, before you even count the 57p daily standing charge. Leaving it running for the eight to ten hours you are out means paying to cool empty rooms.
To be fair, modern inverter units modulate their output and are genuinely efficient at holding a temperature they have already reached, and the pull-down after a hot spell does cost a little more for a short while. But that brief burst when you get home is still far cheaper than cooling an empty flat for half the day. The comparison is not even close.
The smart middle path: schedule, do not run flat out
The answer is not to sweat it out, but to cut the wasted hours. The built-in timer, or a cheap smart plug, lets you start the unit shortly before you get home, so you walk into a cool house without having chilled it for hours while it stood empty. Pre-cooling just before you arrive is the single move that trims the bill without costing you comfort.
Your settings matter too. Nudging the target temperature up towards 26C cuts consumption noticeably compared with lower settings, and running a simple fan alongside it makes the air feel cooler so you can hold that higher setpoint. It is worth reading our air conditioning guide and the wider tips on how to save electricity to see where the savings really sit.
This heat makes air con feel essential, but the gap between an eye-watering bill and a manageable one comes down to habits far more than degrees. Switching it off when you leave, scheduling it for your return and raising the thermostat a notch or two are choices worth tens of pounds by the end of the month. How you run the air con matters as much as the unit itself.