With an amber heat health alert in force and forecasters warning of highs around 35C, portable and plug-in air conditioners are running for hours in homes that were never built for this kind of summer. When the room refuses to cool, the instinct is to grab the remote and push the setting down to 20 or 21C. That single habit is doing more to inflate your electricity bill this week than almost anything else in the house.
Every degree cooler costs you more than you think
The rule of thumb is worth memorising: every degree lower you set the unit raises its energy use by roughly 6 to 8 per cent. That is not a rounding error, it is how the compressor behaves. The colder the target you dial in, the longer that compressor has to keep running to reach it and then hold it.
So the gap between 20C and 26C is not a gentle six notches on the display. Those six degrees translate into a compressor that runs far longer, and the cost stacks up with every hour it stays on. In a poorly insulated flat with the afternoon sun on the windows, the difference widens even further.
What running it all day actually costs
A quick sum shows the stakes. A typical 1 kW portable or monobloc unit, at the electricity unit rate of around 26p per kWh under the Ofgem cap running from 1 July to 30 September 2026, costs about 26p for every hour it runs. Leave it going and that is over 6 pounds a day, and close to 188 pounds a month if it runs around the clock, before you even add the 57p daily standing charge that lands whether the unit is on or off.
The catch is that overcooling does not double your comfort, but it can roughly double how long the compressor works compared with a sensible setting. The headline claim that cooling costs can be cut to a third comes straight from this: overcooling an under-insulated flat at 20C all day at one extreme, and a right-sized unit set to 26C and switched on only when needed at the other.
The setting that keeps you cool for a fraction of the cost
Comfort specialists suggest keeping the gap between indoor and outdoor temperature to about 6 to 8 degrees. With 35C outside, that means aiming for 26 to 27C indoors is both healthier and a fraction of the price of chasing 20C. A violent temperature swing as you walk through the door is hard on the body in any case.
Your best ally in this weather is an ordinary fan. A fan draws only tens of watts rather than the roughly 1 kW an air conditioner pulls, yet the moving air it creates makes the same room feel noticeably cooler. Run a fan alongside the air-con and you can nudge the setpoint up a couple of degrees with no real loss of comfort and next to no added cost.
It also pays to cut the heat coming in: close blinds and curtains through the hottest hours, air the place out overnight and keep the filters clean so the compressor does less work. Before you reach for the remote and push it down, it is worth understanding how the unit behaves and what it adds to your spend. Knowing what your air conditioning really draws is the first step to keeping the bill in check without sweating through the heatwave. Our guide to air conditioning and our tips on how to save electricity are a good place to start.