Units of energy and power

 ‘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.