Does air conditioning use a lot of electricity?

Older fixed-speed units ran at full power until the room was cool, then switched off and on again. A modern wall-mounted split with an inverter slows down instead, but during a hot spell it can still be one of the larger electricity users in your home, simply because it runs for so long.

What it draws while running

A unit's size is its cooling or heating output, not its electricity use. A 2.5 kW unit (about 9,000 BTU) with an example efficiency, or EER, of 4 draws about 0.6 kW of electricity at its rated output. Power and energy: kW and kWh explains the difference between the two figures.

Once the room reaches the temperature you've set, the inverter slows the compressor and the draw drops, often well below the rated figure. It rises again when the sun comes round, a door opens or the weather turns hotter.

Compared with other appliances

Power alone doesn't decide how much electricity something uses, because time matters just as much. The figures below are examples, not measurements of any particular appliance.

The air conditioner draws far less power than the shower or the kettle, but over six hours its total is larger than either. That's why the hours it runs, more than its size, decide whether it adds much to your bill. At an example rate of 25p per kWh — check your own tariff — the 2.4 kWh in the table comes to 60p.

Heating with it

When heating, an air conditioner's efficiency is its COP: the kWh of heat it delivers for each kWh of electricity. A fan heater like the one in the table turns 1 kWh of electricity into 1 kWh of heat. At a COP of 3, an air conditioner delivers the same heat from a third of the electricity.

COP falls as it gets colder outside, so that gap narrows in freezing weather. COP and EER explains why.

What makes use high

• Long hours, such as running day and night through a heatwave.
• A big temperature gap, from a very low setting when cooling or a very high one when heating.
• Heat getting in, from sun on large windows, a room under a hot roof, or doors and windows left open.
• Several units, or a multi-split serving a number of rooms.
• A unit that struggles, because it's too small for the room or its filters are blocked.
• A portable unit. Portable air conditioners generally deliver less cooling for each kWh than split systems, as portable air conditioners explains.

When it's switched off

Switching a system off with the remote or app puts it on standby rather than cutting the power. On standby it draws a small amount of electricity to keep its electronics ready, and in cold weather some outdoor units also use a little to keep the compressor warm. Over a day that's usually small next to the energy used while running.

Switching off at the isolator stops it completely, but check the manual first: some systems need the power back on for several hours before they're used again.

Finding out what yours uses

• The specification sheet gives the power input at rated output, for cooling and for heating.
• The energy label on a split system gives a yearly kWh figure based on standard assumptions. It's useful for comparing models but won't match your own use.
• A smart meter display shows your home's total draw. Note the reading, switch the unit on, and watch the change once the unit settles.
• Some units' apps show an estimate of the unit's energy use.

Fixed units are usually wired in, so plug-in energy monitors won't work on them. Once you have a figure for the unit's average draw, the guide to air conditioning running costs turns it into a cost.