What does air conditioning actually cost to run?

Everyone asks this second, right after the installed price. The honest answer is that it costs less than most people expect to cool a room, and considerably less than most people expect to heat one.

Here are the actual figures, with the arithmetic shown so you can put your own tariff in.

Cooling a bedroom

A 2.5kW wall unit is the standard size for a room up to about 15m². It does not draw 2.5kW of electricity. That number is how much heat it moves, not how much power it uses.

Once the room is down to temperature the unit throttles back and draws roughly 0.3kW. Working hard on a hot afternoon it might pull 0.65kW.

At 25p per unit of electricity:

So a full night of cooling, eight hours with most of it spent ticking over, lands around 60p to 80p.

Heating the same room

This is where it gets more interesting, and where most of the value sits for a British climate.

Air conditioning in heating mode is an air to air heat pump. It moves heat from outside into the room rather than making it. A current unit has a seasonal efficiency of around 4.5, which means one unit of electricity delivers about four and a half units of heat.

That gives you heat at roughly 5.6p per kWh, against:

• 7.4p per kWh from a gas boiler running at 88% efficiency, with gas at 6.5p
• 25p per kWh from any plug in electric heater, because they are all exactly 100% efficient and no better

A fan heater and an oil filled radiator cost the same to run as each other. Neither can beat a heat pump, because they can only ever give you one unit of heat per unit of electricity.

The number that actually matters

The like for like comparison above understates it, because it assumes the boiler is only heating the one room you are sitting in.

In practice, firing the central heating to warm a home office means warming the hallway, the spare room and the kitchen at the same time. Heating one room with a heat pump while the rest of the house stays cold is a completely different bill.

There is a fuller treatment in air conditioning against a gas boiler. On our own gas boiler comparison you can put your room size and your tariffs in and see both numbers. On typical figures, heating one room for a six month season works out around £55 with air conditioning against roughly £291 if the boiler is warming the whole house to do it.

What changes these numbers

• Your tariff. Everything above assumes 25p per kWh for electricity and 6.5p for gas. Both move, and the ratio between them is what matters, not the pennies.
• How well the room holds heat. A solid wall Victorian room loses heat far faster than a modern insulated one, so the unit runs longer.
• What size unit you have. An undersized system runs flat out and never gets there, which costs more than a correctly sized one that cycles.
• Whether it has been serviced. A blocked filter and a dirty coil can add up to a quarter to the running cost. This is the cheapest efficiency you will ever buy back, and it is why annual servicing pays for itself.

The short version

Cooling costs pennies an hour. Heating one room costs less than any other electric option and usually beats running the whole central heating system to warm the same room.

If you want the figures for your specific rooms rather than a worked example, the survey is free and we will put the running costs in the quote.