Danish Wind Industry Assn: "Price per Square Metre Rotor Area matters"

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This is from a page titled: Pitfalls in Wind Energy Cost Analysis
http://www.windpower.org/en/tour/econ/pitfalls.htm

The price per kW Rated Power is a Very Poor Guide to Investment in Wind Power - the Price per Square Metre Rotor Area matters

Many researchers who are interested in the decline in costs of wind power wish to study the decline in the price of wind turbines. They therefore ask for an apparently simple statistic: The price of a wind turbine per kW installed power. That figure is usually difficult to get hold of, and a very poor guide to cost developments for several reasons.
It is very difficult to give a single figure for price per kW installed power, because the price of a turbine varies much more with its rotor diameter than with the rated power of its generator. The reason is that annual production depends much more on the rotor diameter than the generator size. Studies which compare the average price per kW installed power for different technologies are usually misleading, if they include wind power.


Systematic kW Nonsense - an Example

As an example of why it is misleading to use the price per kW rated power for a wind turbine, compare the annual energy production from two machines from the same manufacturer, both mounted on a 50 m tower. (The first one is a high wind machine, the second one a universal machine). You can use the Wind Turbine Power Calculator to verify the results:

1. Vestas V39, a 600 kW turbine with a 39 m rotor diameter
2. Vestas V47, a 660 kW turbine with a 47 m rotor diameter

The result is that annual energy production from the second machine is 45.2% higher than the first machine, despite the fact that the generator is only 10% larger. If you compare the two rotor areas, however, you may observe that the rotor area of the second machine is exactly 45.2% larger than the first machine.
So, if we assume that the price for the second machine is 33% higher than for the first machine you would get very different results, if you compare

1. The price per kW rated power has increased 21%
2. The price per sq m rotor area has decreased 8.4%
3. The price per kWh energy has decreased 8.4%

New wind turbines are increasingly being built with pitch control rather than stall control. This means that the generator size can be varied more freely in relation to the rotor size. In general, there is a tendency to use larger rotor areas for a given generator size. That means that you will get a completely wrong (overestimated) price development when you compare the price per kW installed power for old turbines with new turbines. The relevant price measure is the price per square metre swept rotor area, not the price per kW installed (rated) power.