In many developing countries, the use of small-scale biogas digesters based on animal manure as feedstock is a well-known alternative to using firewood and fossil fuels. The present paper presents the results for a biogas research project in Vietnam where rice straw and water hyacinths have been added as supplementary feedstock.  The project proves that the use of alternative feedstock can be practiced with success and the analysis presents the welfare gains of the technology. The investigation is performed as a social cost-benefit analysis trying to monetize all relevant economic and environmental effects obtained when shifting to climate-friendly biogas produced from a biomass feedstock. The conclusion from the cost-benefit analysis shows the technology to be very attractive in terms of both private and social welfare gains. The net benefits are primarily due to the fuel cost savings (LPG and firewood) and time gains when reducing the usage of firewood.