Hot spells are becoming more common, and the instinct is to reach for air conditioning, which is one of the heaviest electrical loads a home can run. Before you do, a handful of free or near-free habits keep a house markedly cooler, and where you do need help, a fan costs a tiny fraction of what a cooling compressor does. The same logic that governs keeping heat in during winter governs keeping it out in summer.
Block the sun before it gets in
The biggest source of unwanted summer heat is sunlight pouring through glass and warming everything it lands on. The cheapest cooling is to stop it at the window. Close curtains and blinds on the sunny side of the house during the day, particularly south and west-facing windows in the afternoon, and the room stays noticeably cooler. External shading, an awning, a shutter, even a temporary reflective blind, works better still because it stops the heat before it passes the glass. The same heavy curtains that keep warmth in during winter keep the sun's heat out in summer.
Time the windows to the temperature
Opening windows feels like the obvious move, but timing matters. On a hot day the air outside is often warmer than inside by afternoon, so throwing the windows open then lets heat in rather than out. Instead, keep windows and curtains shut through the hottest part of the day to hold the cool, then open them wide in the evening and overnight when the outside air has dropped below the inside temperature, flushing the day's heat out and drawing cool air in. Opening windows on opposite sides of the house creates a through-draught that clears warm air quickly.
Use fans the clever way
A fan does not cool the air; it moves it, and moving air cools you by helping sweat evaporate, so a fan is only worth running when someone is in the room to feel it. Running a fan in an empty room is pure waste. A fan placed to draw cool evening air in through a window, or to push hot air out, helps flush the house. The crucial point on cost is that a fan draws only tens of watts, so running one for hours costs very little, as the running cost calculator will show.
Turn off the heat sources indoors
Your home makes its own heat, and on a hot day it all adds up. The oven and hob throw out a lot, so cook outside, use the air fryer or microwave, or eat cold food on the hottest days. Lights, especially any remaining old bulbs, and electronics left running all give off warmth, so switch off what you are not using. Even drying washing indoors adds heat and humidity. Removing these small internal sources keeps the baseline temperature down for nothing.
The cost gap between a fan and a compressor
This is the number that should give anyone pause before buying a portable air conditioner. A fan uses tens of watts; a portable air conditioner with a compressor draws around a kilowatt or more, perhaps thirty to fifty times as much, so running one through a heatwave can add a serious sum to the bill. Air conditioning genuinely cools the air where a fan cannot, but it is an expensive last resort, not a first move. Exhaust the free measures and the fan first, and reserve real cooling for the rare days when nothing else is enough.
If you do use cooling, use it well
Where air conditioning is genuinely needed, the same discipline that controls any heavy load keeps the cost down. Cool only the room you are in rather than the whole house, shut its door and windows so you are not cooling the outdoors, set the target temperature modestly rather than as cold as it will go, and switch it off when you leave. Combined with shading and night ventilation, occasional, targeted cooling on the worst days costs far less than running a unit flat out because the free measures were skipped.