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The Quarterly Newsletter for the UK New and Renewable Energy Industry

August 1997


The capacity and performance of wind farms are often discussed when considering the merits of wind energy. Richard Boud and Mark Legerton of ETSU take a look at the performance of UK wind farms to date and explain the various terms used.

There are now 43 wind farms operating in the UK (38 if wind farms on adjacent sites are counted as one) with a total installed capacity of 289MW and a corresponding DNC of 120MW. These wind farms have been operating with an average capacity factor of 27%, with availabilities expected to be in excess of 95%.

Statements like these have now been used for a number of years to describe the deployment of wind energy in the UK, but there still appears to be much confusion over what these and other terms actually mean:

  • Installed capacity is simply the maximum steady power that the installed turbine(s) can produce and is dictated by the specification of the generator.
  • DNC is, very broadly, the equivalent capacity of baseload plant that would produce the same average annual energy output as the renewable energy plant. For wind farms the DNC is calculated by subtracting the on-site electrical power consumption from the installed capacity and multiplying the remainder by 0.43 (ie 43%). The reason 0.43 is used, and not the commonly assumed capacity factor of 0.3, is because conventional generating plant also operate at less than their maximum output for much of the time. When DNC factors were originally established it was assumed that wind farms would operate with a capacity factor of 0.3 and conventional plant with a capacity factor of 0.7. The DNC was therefore defined as (0.3÷0.7) or 0.43.
  • The capacity factor, sometimes referred to as the load factor, of a wind farm is the energy produced during a given period divided by the energy that would have been produced had the wind farm been running continually and at maximum output, eg:

Capacity Factor  = electricity production during the period [kWh]
                                 installed capacity [kW] x number of hours in period [h]

Figure 1 shows the variation in the quarterly capacity factor for all normally operating wind farms in England and Wales from 1992 to the end of 1996. Most apparent is the variation in energy production by season, with quarter 1 (January-March) producing roughly twice as much energy as quarter 3 (July-September). The long-term average of 27% is also shown together with the yearly moving average which identifies good and bad years. The annual capacity factors range from 25.6% in 1996 to 29.6% in 1994.

Figure 1


Figure 2 compares the quarterly capacity factors of England, Northern Ireland and Wales throughout 1996. (There are not yet enough data to include Scotland.) 1996 (and 1995) were low wind years right across Northern Europe but, even so, Northern Irelandís wind farms produced an annual capacity factor of 37%. The improved performance of wind farms in Northern Ireland compared with that in England and Wales is likely to be due to a combination of:

  • higher wind speeds
  • taller turbines - wind speed increases with height above the ground
  • improved reliability from more modern machines.

Figures1 and 2 illustrate that capacity factor can vary, for a number of reasons, and that a narrow interpretation of values given as indicative averages can be very misleading.

Figure 2


Capacity factors, which are sometimes mistakenly interpreted as a measure of efficiency, usually vary between 20% and 40% in Europe. This is primarily because of differences in the local wind speed, but they also depend on the design of the wind turbine, particularly the ratio of the rotor swept area to the capacity of the generator. Manufacturers design wind turbines for particular wind conditions, through careful selection of this ratio, such that the cost of energy is minimised. The resultant capacity factors of around 30% are a reflection of this design optimisation and not a measure of the efficiency of the turbines. This can be illustrated by considering a wind turbine with an optimised rotor and generator size. The capacity factor could always be made higher by putting a smaller generator on the wind turbine, so that it ran at full output for a greater proportion of the time. Power generation would then be limited to this lower level at times when the turbine was otherwise capable of producing more electricity, and so the machine would become less cost-effective.

  • The efficiency of any generating plant can be described as electrical energy out divided by the fuel energy in. For example, a modern coal-fired power station converts up to 40% of the chemical energy stored in the fuel into electrical energy. The maximum theoretical efficiency, using current technology, is around 50%. It has been shown that the theoretical maximum of the aerodynamic efficiency of a wind turbine, known as the Betz limit, is 59.3%. In practice, modern wind turbines can achieve efficiencies of 40-50%.

The efficiency is of little importance on its own, as the fuel source (the wind) is free and inexhaustible. Would we be concerned about the fuel economy of our cars if they ran on fresh air?

  • Availability is the proportion of the time that the turbine is available to produce power, including those periods when the turbine is on standby during calms and very high winds. For modern wind turbines, availability is typically 95-98%, which compares favourably with other generating plant with a similar degree of innovation.

These, then, are the main terms used in relation to wind farming. We hope this article will help readers make an educated interpretation of the figures used by all parties in the wind farming debate.

NEW REVIEW is produced by ETSU on behalf of the DTI. Views expressed in the publication do not necessarily represent the views or policies of the Government or the views of ETSU.Neither the DTI nor ETSU endorses any of the products or services featured in NEW REVIEW. Please address correspondence to: Dr Barry Hague, Editor - NEW REVIEW, ETSU, Harwell, Didcot, Oxon OX11 0RA. For more information about the DTI's New and Renewable Energy Programme, contact: New and Renewable Energy Enquiries Bureau, ETSU, Harwell, Didcot, Oxon OX11 0RA; Tel 01235 432450/433601, Fax: 01235 433066