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Good question.  First of all there is no one gas known as biogas.  Biogas can be methane or any other gas that is created from a biochemical reaction.  Be that as it may there is an answer as to your question:

It takes 2300kJ or 0.64kWh to convert 1 litre of water into steam.  How fast that conversion takes place I do not know.  But here is the conversion for calculation how much energy is in water:

E = c_p dt m             (1)

where

E = energy (kJ, Btu)

c_p = specific heat capacity (kJ/kg^oC, Btu/lb ^oF) (4.2 kJ/kg^oC, 1 Btu/lb_m^oF for water)

dt = temperature difference between water stored and the surroundings (^oC, ^oF))

m = mass of water (kg, lb_m)

Or graphically:

 

But at least you know part of your question.  Why do you need to know?