## Power and Energy

It is easy to confuse power and energy. Power is a measure of the flow of energy and is measured in kilowatts (kW). Energy is the product of power and time and is measured in kilowatt-hours(kWh). The units shown on your electricity bills are in fact kWh.

eg. a turbine generating 3 kW continuous output for 20 hours will have produced 60 kWh(units) of electrical energy.

eg. A kettle with a 2 kW rating will consume 0.2 kWh if it runs for 6 minutes(1/10th of an hour).

Wind turbines are often described in terms of the maximum power they can produce eg. 6 kW, 11 kW, 25 kW. However this figure on its own can be misleading as the actual power produced by a turbine varies with the wind speed. Manufacturers therefore normally give additional information in the form of a table or a graph called a ‘power curve’ showing the turbine performance for a range of wind speeds.

The annual energy yield that you can obtain from a turbine depends on the shape of its power curve and the range of wind speeds obtained at your site throughout the year. When comparing turbines therefore, it is not always the case that the turbine with the highest power rating will generate the most energy.

Wind energy capture is strongly related to the amount of air intercepted by the turbine rotor eg. a 6m radius rotor sweeps an area 4 times greater than a 3m radius rotor. Bigger rotors therefore capture more wind and generate more energy.