A quick and dirty calculation: I bought 28 shares from OX2 windpower coop, which entitles me to 28,000 kWh at cost price. Which is what our house/home office uses per year. The price is about €700/share. But I bought shares from the market (people that want to sell their shares), which for some reason are cheaper. I paid about €590/share.
Over the last seven years (I haven't had my shares that long), the saving on cost of electricity, which is tax free, would have been about €960/year. Which is about 5.7% ROI /year. Better than bank rates but worse than index linked stock market investments (I think).
http://www.ox2.com/en/wind-power/private-users/
I don't think OX2 offer electricity in Germany, but maybe in Finland and Poland. Even if they did, it would also depend on how the German regulator has structured how this can work, as well as how taxation on this is structured.
