How much does air conditioning cost to run?

There is no single running cost for air conditioning, because the same unit can tick over gently on a mild evening and work hard all through a heatwave. What you can do is estimate the cost for the way you expect to use it, with figures you can find and check yourself.

The method

Running cost comes down to two numbers: how much energy the unit uses, in kilowatt-hours (kWh), and what you pay for each kWh.

1. Find the unit's power draw in kilowatts (kW).
2. Estimate its average draw while it runs. This is usually lower than the rated figure, for reasons explained below.
3. Multiply the average draw by the hours of use to get kWh.
4. Multiply the kWh by your unit rate.

For example, a unit averaging 0.5 kW for 6 hours uses 3 kWh. At an example rate of 25p per kWh — check your own tariff — that comes to 75p.

Your unit rate is on your bill or in your supplier's app, in pence per kWh. Leave the daily standing charge out of the sum, because you pay it whether the unit runs or not. If your rate changes through the day, price each period separately, as time-of-use tariffs explains.

Finding the power figure

The size in a unit's description is not its electricity use. A unit sold as 2.5 kW (about 9,000 BTU) delivers that much cooling but draws only a fraction of it in electricity, because it moves heat rather than making it. Power and energy: kW and kWh explains the difference.

The label's yearly figure is useful for comparing models, not for predicting your bill, because your hours and weather won't match its assumptions. Some specification sheets also give minimum and maximum power figures alongside the rated one.

Fixed units are usually wired in, so a plug-in energy monitor won't work. To use a smart meter display instead, note the reading, switch the unit on, and watch how the reading changes once the unit settles.

Why the average is lower than the rating

Most wall-mounted units use an inverter, which lets the compressor speed up and slow down. The unit works hardest while it brings a hot or cold room to temperature, then slows down to hold it there, usually drawing much less.

Draw is highest when:

• the room starts a long way from the temperature you've set
• it's very hot outside when cooling, or very cold when heating
• sun is shining through the windows when cooling, or doors and windows are open

When heating, the electricity used is the heat the room needs divided by the unit's efficiency, its COP. Efficiency falls as it gets colder outside, so the same amount of heat takes more electricity on a frosty day than on a mild one. COP and EER explains how that works.

Worked examples

The averages below illustrate the method; they are not measurements. Your own unit, room and weather will give different figures, and the rate is only an example, so check your own tariff.

To estimate a whole season, multiply by the number of days you expect to use it. Thirty nights like the bedroom example come to 72 kWh, or £18 at the same example rate.

What pushes the cost up or down

• The temperature you set. The bigger the gap between the room and the air outside, the harder the unit works.
• Hours of use. The cost rises with every hour the unit runs.
• Heat getting in or out. Sunny windows, poor insulation, draughts and open doors all add to the work.
• The unit's efficiency. The seasonal ratings on the energy label, SEER for cooling and SCOP for heating, let you compare models.
• Size. A unit too small for the room runs flat out for long spells, while one far too big can keep stopping and starting.
• Upkeep. Blocked filters and a cluttered outdoor unit make the system work harder.

Most of these are within your control, and how to cut running costs goes through practical changes.