How can you model a biogas-powered electric generator in HOMER?

There are two methods to model a biogas-powered electric generator in HOMER. For both methods, first add a generator to the schematic. 

 1. If there is an abundant supply of biogas feedstock throughout the year, create a new fuel by clicking the Fuel tab in the Generator Inputs window and then clicking the "New" button. To create the new fuel, you will need to know the following characteristics of the biogas fuel: lower heating value, density, carbon content and sulfur content. Carbon and sulfur content are only necessary if you are interested in modeling the emissions of the system.

2. If there is a constrained supply of feedstock, choose Biogas as the fuel on the Fuel tab in the Generator Inputs window. That will cause a biomass resource button to appear in the Resource section of the schematic where you can indicate the availability of the biomass resource throughout the year.

Additionally, the help file has several entries describing how to use HOMER to model biogas power systems.  Try searching for the term "biogas" under Help.