The electrical power of a wind turbine can be calculated with formula 4.2 of my public report KD 35. So even if someone has no aerodynamic knowledge, he can check if the power as specified by a supplier is realistic by putting realistic values of the Cp and the efficiency of generator and transmission in this formula. A big problem is that most suppliers only mention the peak power at a very high wind speed. This is especially the case for wind turbines which have no safety system which limits the power, the rotational speed and the thrust at high wind speeds. So this extremely high peak power has not only the disadvantage that it is supplied only for some hours a year but also that a wind turbine without a proper safety system can be very dangerous. A very high peak power also has as disadvantage that all electric components behind the generator like the rectifier, the cables, the voltage controller and the dumpload must be very large other wise they will be blown during a strong wind gust.
What you need to judge the usefulness of a wind turbine in a certain wind regime, is the Pel-V curve. This curve gives the electrical power as a function of the wind speed. This curve starts at the so called cut-in wind speed. The rotor starts rotating at the starting wind speed when the starting torque of the rotor is larger than the cogging torque of the generator. If the generator has a large peak on the cogging torque, the starting wind speed can be much higher than the cut-in wind speed resulting in hysteresis in the Pel-V curve. So if the cut-in wind speed is 3 m/s and if the starting wind speed is 5 m/s, you only get power at 4 m/s if the wind speed has been higher than 5 m/s in the recent past. This is another reason why people can be very disappointed about the power output of their wind turbine if they live in a region with rather low wind speeds.
All VIRYA wind turbines which I have designed have an effective safety system and for all public designs I give a Pel-V curve. These curves are given in folders which can be found at the menu VIRYA-folders at my website:
www.kdwindturbines.nl. So if you want to buy a commercial wind turbine, be sure that the supplier supplies a Pel-V curve. If this curve isn't given, you will be disappointed.