The wind speed is always fluctuating, and thus the energy content of the wind is always changing.

Exactly how large the variation is depends both on the weather and on local surface conditions and obstacles.

Energy output from a wind turbine will vary as the wind varies, although the most rapid variations will to some extent be compensated for by the inertia of the wind turbine rotor.

In most locations around the globe it is more windy during the daytime than at night. The graph to the left shows how the wind speed at Beldringe, Denmark varies by 3 hour intervals round the clock. (Information from the European Wind Atlas).

This variation is largely due to the fact that temperature differences e.g. between the sea surface and the land surface tend to be larger during the day than at night. The wind is also more turbulent and tends to change direction more frequently during the day than at night.

From the point of view of wind turbine owners, it is an advantage that most of the wind energy is produced during the daytime, since electricity consumption is higher than at night. Many power companies pay more for the electricity produced during the peak load hours of the day (when there is a shortage of cheap generating capacity). We will return to this subject in the section on Wind Turbines in the Electrical grid.

We treat this subject in the section on Wind Turbines in the Electrical grid.