It has been one of the hottest summers on record, with heatwave after heatwave from June into August. For weeks, fans ran non-stop and portable air conditioners hummed through the afternoons, and the cost of it lands with the next bill. It is worth working out now what keeping cool really added, so the surprise is not saved for the final figure.

How much cooling actually uses

A portable air conditioner, the most common sort in British homes, typically draws between 1 and 1.5 kWh for every hour it runs. Leave it on for six hours a day and that is easily 7 to 9 kWh just to cool one room. At around 26p per kWh, that is close to £2 a day, which over a full month of heat becomes £50 to £60 on top of the usual bill. A fixed split unit is more efficient, but far fewer homes here have one. A humble fan, by contrast, costs only pence a night. And anyone who left the unit on overnight to sleep through the amber-alert nights, as many did, will have seen that daily cost quietly double.

The bill for a whole season

Across June to August, a household that leaned on a portable air conditioner through the hot spells can find £120 to £180 added to its bills compared with a cooler summer. That single item explains most of the summer rise, far more than the fridge, the lights or small appliances. The figure swings a lot with how well the home is insulated, its aspect and the temperature you set. For a fuller picture of running costs, our guide to air conditioning breaks it down.

Where you could have saved, and still can

The golden rule is temperature: every degree lower on the thermostat pushes consumption up by roughly 6 to 8%. Setting a unit to 24 degrees rather than 20 makes a real difference to the end-of-season total, and you barely notice it, because what matters is the gap with the air outside. Clean filters help, as does keeping doors and windows shut while the unit works, drawing curtains or blinds on the sunny side, and reaching for a fan first whenever the heat allows. These are the same habits set out in our guide to saving electricity, applied to cooling.

A useful check before winter

If the summer bills stung, now is a sensible moment to see whether your tariff still stacks up, before the heating season piles more onto your usage. Reading your bill closely with our guide to energy bills explained helps you spot what you are paying for each kWh, both the summer you have just had and the winter on the way.