Air conditioning guide
Air conditioning running costs
Running cost is the question we get asked most and the one generic figures answer worst. Here is how to calculate it for your house instead.
Why published running costs rarely apply to you
A running cost figure is the product of four things: how much power the unit draws, how many hours it runs, how hard it is working while it runs, and what you pay per unit of electricity. Change any one and the answer changes.
Two identical units in two houses can differ by a large multiple simply because one is in a shaded north-facing bedroom used at night and the other is in a south-facing conservatory used all afternoon. That is why we would rather show you the arithmetic than a number.
The calculation
You need three figures, and you can get all of them before you buy anything.
1. The unit’s power input, in kilowatts
This is not the same as its cooling capacity. A system’s cooling output and its electrical input are two different numbers, and the one you pay for is the input. Both are on the manufacturer’s specification sheet. Ask your installer for the rated power input of the specific model being quoted.
2. How many hours a day you will actually run it
Be realistic rather than worst-case. Most domestic cooling is seasonal and intermittent — a handful of weeks a year, often only part of the day or overnight. Estimating "hours per day across the weeks I would use it" gets you closer than assuming constant use.
3. Your electricity rate, in pence per kilowatt hour
Take it from your latest bill or your supplier’s app rather than a national average. If you are on a time-of-use tariff, note that overnight bedroom cooling may fall in a cheaper window, which materially changes the answer.
One important adjustment: an inverter system does not draw its rated input continuously. It works hardest bringing the room down to temperature, then modulates to hold it. Treating the rated input as constant gives you a deliberately pessimistic figure, which is a reasonable way to sanity-check affordability.
What makes the running cost go up
- An undersized system, which runs at full tilt for longer instead of settling
- Rooms with high solar gain and no shading — conservatories especially
- Setting the target temperature far lower than you actually need
- Leaving doors open to spaces you are not conditioning
- Dirty filters, which restrict airflow and make the system work harder
- An outdoor unit with restricted airflow, boxed in or blocked by planting
The first two are decided at specification and survey. The rest are within your control afterwards, and filter cleaning is the one people forget.
Efficiency ratings, and what they actually tell you
Air conditioning systems carry seasonal efficiency ratings covering cooling and, where applicable, heating. They are useful for comparing one model against another on a like-for-like basis, which is exactly what they are designed for.
What they are not is a prediction of your bill. They are calculated against standardised assumptions, and your house is not standardised. Use them to choose between two quotes; use the calculation above to work out what you will actually pay.
Using it for heating
Most modern domestic systems are air-to-air heat pumps, which move heat rather than generating it. That makes them efficient compared with electric resistive heaters such as fan heaters or oil-filled radiators, because each unit of electricity moves more than one unit of heat.
Whether that is cheaper than your existing central heating depends on what you currently heat with and what you pay for it, since you are comparing an electricity rate against a gas rate. Heating a single room you use, rather than the whole house, is often where the sums work best.
Heat pump heating output also falls as the outside temperature drops, so performance in mild weather and performance in a cold snap are not the same. If winter use matters, ask the installer for the heating output at a low outdoor temperature rather than the headline figure.
Maintenance and its effect on cost
Filters need cleaning regularly and it is a job you can do yourself. A blocked filter restricts airflow, which reduces performance and pushes consumption up for the same result.
Beyond that, systems generally benefit from periodic servicing, and some manufacturer warranties require it at set intervals to remain valid. Ask what the warranty conditions are before you buy, not after — it is part of the running cost.
Common questions
Is air conditioning expensive to run?
It depends almost entirely on how much you use it and what you pay per kWh. Domestic cooling in the UK is seasonal and intermittent, so the annual cost is often lower than people expect, but a conservatory running all afternoon through a hot spell is a very different figure from a bedroom unit used overnight for two weeks. Use the calculation above with your own rate.
Is it cheaper to run air conditioning or a fan?
A fan uses far less electricity, because it moves air rather than removing heat. It also does not lower the room temperature. They solve different problems, and for a genuinely hot room a fan is not a substitute.
Is using air conditioning for heating cheaper than my central heating?
Possibly, but it is a comparison between an electricity rate and whatever you currently heat with. A heat pump moves more heat than the electricity it consumes, which makes it efficient in its own terms. Whether that beats your boiler depends on both tariffs and on whether you are heating one room or the whole house.
Does leaving it on all day cost less than switching it on and off?
For a room you are actually using throughout the day, holding a steady temperature can be more efficient than repeatedly pulling it down from hot, because the system spends more time modulating and less time at full output. For a room you use for a few hours, running it only when you need it is cheaper. Timers are useful for exactly this.