Before you leave something running all night, it helps to know what it adds to the bill. The gap between a fan and a portable air conditioner is big enough to change which one you reach for. Here is what each costs to run, with the simple sums so you can check your own model. The figures below assume electricity at about 25 pence per unit (kWh).
The quick numbers
| Appliance | Typical power | Per hour | For 8 hours |
|---|---|---|---|
| Fan | 30 to 60 W | 1 to 2p | about 10p |
| Dehumidifier | 150 to 300 W | a few pence | 20 to 40p |
| Portable air conditioner | 900 to 1,400 W | 21 to 37p | up to about £2.40 |
Work it out yourself
The sum is simple. Take the wattage on the label and divide by 1,000 to get kilowatts. Multiply by the hours you will run it to get units (kWh). Then multiply by your price per unit. A 1,200 watt air conditioner run for 8 hours is 1.2 kW times 8, which is 9.6 units, times 25 pence, so about £2.40 a night.
Fans: pennies a night
Fans barely register on the bill. At 30 to 60 watts they cost roughly 1 to 2 pence an hour, so a full night runs to about 10 pence. They do not cool the air, but for most UK nights moving air over your skin is enough. See the best fans for 2026.
Dehumidifiers: cheaper than they look
A dehumidifier draws a couple of hundred watts when the compressor is on, but it does not run flat out. The humidity sensor switches it off once the air hits the target, so the real running cost is usually a few pence an hour. A drier room also feels cooler, which can save you reaching for the air con at all.
Portable air conditioners: the pricey one
This is the big draw on the meter, because the compressor is doing real cooling work. Expect 21 to 37 pence an hour, or about £50 to £88 a month with regular use. An A-rated unit costs the least to run, and it cycles down once the room reaches temperature. Our pick of the best portable air conditioners flags the efficient ones.
How to keep the cost down
Match the unit to the room rather than buying the biggest one, since an oversized air conditioner just costs more to run. Use the timer and eco modes, and keep doors and windows shut while the air con is on so it is not fighting the whole house. For fans and dehumidifiers, the lowest setting that does the job is always the cheapest.
Common questions
Is it expensive to leave a fan on all night?
No. A typical fan uses 30 to 60 watts, which is roughly 1 to 2 pence an hour, or about 10 pence for a full night.
How much does a portable air conditioner cost to run?
Around 21 to 37 pence an hour at current prices, or about £50 to £88 a month with regular daily use. A-rated models and matching the unit to the room keep it at the lower end.
Do dehumidifiers use a lot of electricity?
Less than people expect. They draw a couple of hundred watts while running, but the humidity sensor switches them off once the air is dry enough, so the real cost is usually only a few pence an hour.
What uses the least electricity?
A fan, by a wide margin. A dehumidifier sits in the middle. A portable air conditioner uses the most, because it has a compressor doing the actual cooling.