10 cows will give/day 200/Kg of cow dung, this in a bio digester will yield about 6 M3 of Gobar gas. Which means about 0.03 m3/ kg of cow dung.
I read that the biogas consumption was 0.6 m³ (at standard atmospheric conditions) when 1 kw · hr electricity was generated.
So I guess you can do the math and get what you need.
Nikhil