Units of energy and power

Energy’ and ‘power’ are often used interchangeably.

 ‘Energy’ and ‘power’ are often used interchangeably, but they do mean different things and they are measured in different units.

Power – the rate of energy conversion – is commonly measured in Watts (W), kiloWatts (1 kW = 1,000 Watts) or MegaWatts (1MW = 1,000,000 Watts).

Energy – the ability to do work – is commonly measured in Watt-hours (Wh), kilowatt-hours (kWh) or megawatt-hours (MWh).

Think of a 100W light bulb left on for an hour. Its power rating is 100 Watts and it uses 100 Wh of energy in one hour. A much more efficient option would be a low-energy light bulb: a 20W low-energy bulb can give the same amount of light but uses only 20Wh of energy in an hour.