When they talk about wind turbines, the quoted power of a turbine is the peak power, the most it can generate when the wind is strongest. The power output of a turbine varies as the cube of the wind speed. That means that if the wind drops from 20 miles per hour (9 m/s) to 10 mph (4.5 m/s), the electric power produced goes down by a factor of eight. The average number of megawatts generated is then much lower than the maximum that the turbine is built for. Figure 3.5 gives an idea of how variable wind power is. The data are from the area controlled by E. ON Netz, a large company that controls 40% of Germany’s wind capabilities. During the year, this example shows that the power varied from 0.2 to 38% of the peak grid power! For this reason, a turbine built to generate 5 MW actually produces much less than that on the average. Just how much is shown in Fig. 3.6. This graph shows how much time during the year the wind power generated in a certain area was the number of gigawatts shown on the vertical scale. The time is measured in quarter-hours. We see that the maximum installed capacity of 7 GW was never reached, and even 6 GW was produced for a very short time. The average power over the year was less than one-fifth of the installed power capability. For half the year, less than 14% of the installed capacity was usable.11 So 7 MW can mean only 1.3 MW!

We often see statements like “The 5-MW titan [in Denmark]…will average enough power for 5,000 homes,”8 or “The 108 [1.5-MW] turbines. in the Colorado Green project.. .produces roughly enough electricity each year to supply more than 52,000 homes4.” The first averages out to 1 kW (peak) per home, while the second works out to be 3.1 kW (peak) per home. Clearly, this number will depend on the amount of wind at each locale as well as the lifestyle there in terms of electricity use.

In 2001, the yearly average electricity consumption in the USA was 1.2 kW per home12 or 0.47 kW per average person. This is on a steady basis, averaged over a whole year. Now, 1.2 kW goes into 1 MW (=1,000 kW), 833 times. So if average wind power is only 20% of the peak power, as we found above for Germany, 1 MW would supply only a fifth of 833 or 166 homes. This is a little less than the number of 250-300 homes quoted in footnote 4, but the discrepancy can be accounted for if steadier winds were assumed. The Colorado example above works out to give an average-to-peak wind factor of 38%, just twice the number for Germany. This means that 1 MW of peak power in Colorado would supply 320 homes, in good agreement with the quoted number of 250-300 homes for the US average. In the Denmark example above, 1 MW of peak power would provide average power for 1,000 homes, about three times the number in the Colorado case. It is quite possible, however, that electricity is used much more sparingly in Denmark than in the USA.

In summary, the average power from wind turbines is only 19-38% of the installed power capability, depending on the location. The number of homes a wind farm can power also depends on the energy usage pattern in that location. Consequently, claims about the efficiency of wind farms can vary widely and can­not always be believed without checking the facts.