kW and BTU explained

You'll meet both units when you shop for air conditioning. Kilowatts appear on energy labels and most specification sheets for fixed systems, while BTU figures are common on portable units and in some product names. They describe the same thing, so converting between them is simple once you know what the numbers mean.

What the numbers measure

Both describe capacity: the rate at which an air conditioner moves heat out of a room when cooling, or into it when heating.

• kW (kilowatt) is a thousand watts, the standard metric unit of power.
• BTU (British thermal unit) is an older unit of energy, roughly the heat needed to warm one pound of water by one degree Fahrenheit.

Strictly, an air conditioner rated in BTU is rated in BTU per hour, because capacity is a rate. Labels and adverts usually drop the "per hour", but that's what they mean.

Converting between them

One kW is about 3,412 BTU per hour. To convert, multiply kilowatts by 3,412, or divide BTU by 3,412. Sizes are rounded to familiar numbers when units are sold, so treat a BTU figure as a size class rather than an exact measurement.

Some model codes include a size number. A number such as 25 or 35 often refers to the kW class (2.5 or 3.5 kW), while 09, 12 or 18 often means thousands of BTU. It isn't a reliable rule, so check the specification sheet for the actual capacity. For how to choose a size in the first place, see what size air conditioner you need.

Cooling, heating and the range in between

A specification sheet for a split system usually gives more than one capacity figure.

• Cooling and heating are listed separately, and the heating figure is often a little higher.
• Rated capacity, sometimes called nominal capacity, is measured under standard test conditions.
• Minimum and maximum figures show the range an inverter unit can run across, slowing down when the room needs little and speeding up when it needs more.

Real output depends on the weather. Cooling output drops on very hot days and heating output falls as it gets colder outside, so the rated figure is a fair basis for comparing units rather than a promise for every day of the year.

Capacity isn't electricity use

The capacity figure is the heat moved, not the power drawn from the mains. A heat pump moves heat rather than making it, so it usually needs much less electricity than the heat it delivers. Specification sheets list the electricity separately, as power input.

Dividing one by the other gives the efficiency at those test conditions. For example, a unit that moves 3.5 kW of heat while drawing 1 kW of electricity has a ratio of 3.5. For cooling this ratio is called EER and for heating COP, and COP and EER covers both in more detail.

Running costs are worked out in kilowatt-hours: the average power drawn multiplied by the hours it runs. If a unit draws an average of 0.5 kW over four hours, it uses 2 kWh. At an example rate of 25p per kWh (check your own tariff), that comes to 50p. Power and energy: kW and kWh explains the difference between the two.

BTU and portable units

Portable air conditioners are often sold by BTU, and their figures aren't directly comparable with a split system's. A single-hose portable unit blows warm air out through its hose, and warmer air from outside leaks back into the room to replace it, so it usually cools a room less than its rating suggests. See portable air conditioners for how they work and where they make sense.