I have been using a Mobile Logger from Logic Energy to collect data on a 3 metre diameter turbine close to my house. The turbine is built to the Recipe Book. In the gallery you can see the site and the measurement mast, which comes from Radioworld.
The data is interesting. There is a different power curve for south wind coming up the slope from the north wind blowing down the hill to the turbine. The tower is accurately vertical. This is an example of how site-dependent a power curve is. It depends on many factors including the particular machine, actual battery voltage, cable resistance, and even the wind direction in most cases.