I've done some calcs that show that a 1.75m diameter turbine is only likely to produce about 1,100kWh with a mean annual windspeed of 5.5m/s. This is unlikely to be the sort of speed that anyone in a built-up area is going to get on their roof. The figure drops to about 300kWh with a mean speed of 3.5m/s. (This is based upon a conversion efficiency of 35% - which is what small turbines have been found to achieve when tested.)
There has been talk of independant testing and government certification for such claims but nothing has transpired yet. Some system along the lines of the mpg figures for cars is needed and needed quickly.
B&Q are now selling the Windsave turbines (not sure if any have actually been delivered yet though) "subject to survey". I would love to know how they determine the windspeed on peoples' rooftops. If they are just using the DTIs NOABL figures then I think they will be little better than meaningless in urban areas.
The whole situation could end-up giving the domestic renewables industry the same sort of reputation as enjoyed by double-glazing salesmen.