This type of question is asked for many years. Search google scholar for "Biogas energy balance"

https://scholar.google.com/scholar?q=biogas+energy+balance

Anaerobic digestion has two types: mesophilic and thermophilic. Thermophilic uses heat and the amount of input energy is the decision of the operator. If you're using electricity then you can determine from the rated wattage of the heater, or by measuring the current and voltage using meters to determine the electric power consumption. For mesophilic, basically no energy is used unless you count in auxiliary things such as transportation etc., which may vary depending on local conditions.

Dear Giovanni Ferrari

I hope you are doing well. You won't need electricity if you do not have a CSTR anaerobic digester. In case you do have a CSTR digester, you will need electricity to stire the content of the digester on a regular basis, which is usually negligible compared to the heat demand of the digester (less than 5%), but you should absolutely calculated the heat demand of the digester to make sure you are providing adequate amount of energy for the digestion process. In order to calculate, you should take into account the required heat to increase the feedstock temperature to the operation temperature of the digester (sensible heat), the heat loss through the ceiling, walls, and pipes. The sum of the mentioned heat flows would result in the total heat demand of the digester. I am sure this will solve your problem.

Regards

Yoram Gerchman Saba Gholizadeh Shaharin Anwar Sulaiman

Thank you for your answers. I am modelling an LCA for some different biomethane production systems, and it is the first time I try this type of analysis.