Power and energy: kW and kWh

Mixing up the two is an easy way to get a running-cost estimate badly wrong. Air conditioning adds a twist of its own, because the kW figure in a unit's description is the heat it moves, not the electricity it uses.

Power is a rate, energy is an amount

Think of a car journey. Speed, in miles per hour, is how fast you're going at any moment; distance, in miles, is how far you've gone. Power and energy work the same way: kW is the rate, kWh is the total.

• 1 kilowatt (kW) is 1,000 watts (W).
• 1 kilowatt-hour (kWh) is the energy used by something drawing 1 kW for one hour.
• Energy in kWh = power in kW × time in hours.

To turn watts into kilowatts, divide by 1,000: a 500 W appliance draws 0.5 kW.

Each row below uses exactly 1 kWh:

Something powerful used briefly can use less energy than something modest left on for hours. A 3 kW kettle boiling for 2 minutes uses 0.1 kWh, the same as a 10 W light bulb left on for 10 hours.

The kW figures on an air conditioner

An air conditioner's specification sheet gives more than one kW figure, and they measure different things.

• Capacity is the cooling or heating the unit delivers, such as 2.5 kW (about 9,000 BTU) or 3.5 kW (about 12,000 BTU). This is the size people talk about. BTU figures on air conditioners are really BTU per hour, another unit of power, and 1 kW is about 3,412 BTU per hour. kW and BTU explained covers the conversion.
• Power input is the electricity the unit draws to deliver that capacity. It's usually much smaller, because the unit moves heat from one place to another rather than making it.

The ratio between the two is the unit's efficiency. A unit delivering 3.5 kW of cooling from 1 kW of electricity has an efficiency of 3.5, and COP and EER explains how these figures are measured.

Power input also changes as the unit works. The rated figure applies at the rated output under test conditions. An inverter unit draws more while it works flat out and less once it slows down to hold the temperature.

Reading your bill and meter

Your electricity bill charges for energy, not power. You'll see:

• a unit rate in pence per kWh (a kWh is often simply called a "unit")
• a standing charge in pence per day, paid however much you use

A smart meter's in-home display usually shows both measures: what your home is drawing right now, and how much energy it has used so far today.

To turn a power figure into a cost per hour, multiply the kW by the unit rate. An air conditioner drawing 0.5 kW, at an example rate of 25p per kWh — check your own tariff — costs 12.5p for each hour it draws that much. How much does air conditioning cost to run? takes the method further.

Gas is billed in kWh too, although the meter measures volume. Your supplier converts the reading to kWh using the gas's energy content, so gas and electricity prices per kWh can be set side by side. What each kWh delivers as heat still depends on the appliance using it.

Common mix-ups

When a figure looks surprisingly large or small, check whether it's a rate (kW) or an amount (kWh) before doing anything else with it.