When the heat refuses to break even at night, millions of us fall back on the first cooling tips passed on by family or social media. The problem: several of these reflexes cool nothing at all, inflate your electricity bill, or even make the discomfort worse. Here are the four most common, what the physics says, and what they cost.
1. The fan left running in an empty room
A fan doesn't lower the temperature: it cools people, by evaporating sweat and carrying heat away from the body. In an empty room it cools precisely nothing; in fact its motor adds a whisper of heat to the air. Leaving it on «to keep the bedroom cool» while you're at work is pure spend for zero benefit. Simple rule: the fan goes on when someone is in front of it, and off when you leave.
The sums are quick: a 50W fan forgotten for eight hours a day uses 0.4 kWh, about 10p, or £3 a month spent cooling an empty room. The one sensible exception is a fan sat in the window in the evening, facing outwards: that genuinely does speed up the room's air exchange.
2. The portable air conditioner used like a fan
This is the one that costs more than everything else combined. A 1,000W portable unit left running around the clock draws up to twenty times more than a fan. At the current price cap rate of 26.11p per kWh, that is about 26p an hour, £6.27 a day, and close to £188 over a month of non-stop use. Worse, with the exhaust hose badly fitted or the window cracked open around it, the unit pulls part of the heat it expels straight back in. If you use one, use it properly: room closed, hose sealed at the window, a few targeted hours in rooms that are actually occupied.
3. Windows thrown open in the middle of the day
«Letting some air in» at 3pm on a 33°C day is the fastest way to let the heat in. If it is hotter outside than inside, every open window is working against you. Ventilation belongs to the hours between sunset and dawn, when the outside air finally drops below the inside temperature, ideally as a through-draught, with openings on two opposite sides. During the day: curtains drawn and windows shut on the sunny side.
4. Ignoring the little radiators running inside your home
The oven at lunchtime, a desktop PC on all day, set-top boxes and consoles on standby, halogen bulbs: every watt consumed indoors ends up as heat. One by one they look like details; added together, in a closed two-bed home, they are worth a degree or two. Cook in the evening, switch things off properly, and swap to LEDs: it is the only «air conditioner» that works by turning things off rather than on.
Worth remembering: fans cool bodies, not rooms; a portable aircon running 24/7 costs close to £188 a month at the price cap; windows open at night, not at 3pm; and every appliance left on is a small radiator. Before buying anything against the heat, it pays to eliminate whatever is producing it.
So what actually works?
The effective strategies are well known and almost all free: a home sealed by day and cross-ventilated by night, a fan aimed at people, evaporation (a wet sheet at the window after dark), and no heat produced indoors during the critical hours. And if air conditioning really is unavoidable, choose an efficient unit, set it to 25–26°C, and remember: every extra degree you demand of the thermostat is paid for, month after month, on your bill.