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Windspeed data can be obtained from wind maps or from the meteorology office. Unfortunately the general availability and reliability of windspeed data is extremely poor in many regions of the world. However, significant areas of the world have mean annual windspeeds of above 4-5 m/s (metres per second) which makes small-scale wind powered electricity generation an attractive option. It is important to obtain accurate windspeed data for the site in mind before any decision can be made as to its suitability. Methods for assessing the mean windspeed are found in the relevant texts (see the ‘References and resources’ section at the end of this fact sheet).

The power in the wind is proportional to:

• the area of windmill being swept by the wind

• the cube of the wind speed

• the air density – which varies with altitude

The formula used for calculating the power in the wind is shown below:

**P = ½.ρ.A.V ^{3}**

where, P is power in watts (W)

ρ is the air density in kilograms per cubic metre (kg/m^{3})

A is the swept rotor area in square metres (m^{2})

V is the windspeed in metres per second (m/s)

The fact that the power is proportional to the cube of the windspeed is very significant. This can be demonstrated by pointing out that if the wind speed doubles then the power in the wind increases by a factor of eight. It is therefore worthwhile finding a site which has a relatively high mean windspeed.

**Wind into watts**

Although the power equation above gives us the power in the wind, the actual power that we can extract from the wind is significantly less than this figure suggests. The actual power will depend on several factors, such as the type of machine and rotor used, the sophistication of blade design, friction losses, and the losses in the pump or other equipment connected to the wind machine. There are also physical limits to the amount of power that can be extracted realistically from the wind. It can been shown theoretically that any windmill can only possibly extract a maximum of 59.3% of the power from the wind (this is known as the Betz limit). In reality, this figure is usually around 45% (maximum) for a large electricity producing turbine and around 30% to 40% for a windpump, (see the section on coefficient of performance below). So, modifying the formula for ‘Power in the wind’ we can say that the power which is produced by the wind machine can be given by:

**P _{M} = ½.Cp.ρ.A.V^{3}**

where,

P_{M} is power (in watts) available from the machine

C_{p} is the coefficient of performance of the wind machine

It is also worth bearing in mind that a wind machine will only operate at its maximum efficiency for a fraction of the time it is running, due to variations in wind speed. A rough estimate of the output from a wind machine can be obtained using the following equation;

**P _{A} = 0.2 A V^{3}**

where,

P_{A} is the average power output in watts over the year

V is the mean annual windspeed in m/s

There are two primary physical principles by which energy can be extracted from the wind; these are through the creation of either lift or drag force (or through a combination of the two). The difference between drag and lift is illustrated by the difference between using a spinnaker sail, which fills like a parachute and pulls a sailing boat with the wind, and a Bermuda rig, the familiar triangular sail which deflects with wind and allows a sailing boat to travel across the wind or slightly into the wind.