Capacity is the maximum amount of electrical power a system can handle. Load is the amount of power being used at a certain time, the actual instantaneous consumption, while demand refers to the peak amount of power required at a certain time. These are measured in watts, kilowatts (1,000 watts) or megawatts (1,000 kilowatts) and annual demand governs planning of the UK’s infrastructure.
Energy is the supply of power over a period of time and is usually measured in watt hours (Wh) – the amount of power used or produced in an hour (kWh is commonly used for home applications whereas the grid might use Mega Wh – 1MWh=1,000kWh, Giga Wh - 1GWh = 1m kWh, and Tera Wh – 1TWh = 1m MWh). A UK household typically consumes 2,700 kWh every year. An offshore wind turbine can produce more than 8m kWh a year.