In this paper, we analyse the role of biogas and biogas-derived fuels in a 100% renewable energy system for Denmark using the energy system analysis tool EnergyPLAN. The end-fuels evaluated are biogas, biomethane and electromethane. First, a reference scenario without biogas is created. Then biogas, biomethane and electromethane replace dry biomass-derived fuels in different sectors of the energy system. The results show that biogas and biomethane reduce dry biomass consumption by up to 16% when used for power, heat or industrial sectors. If biogas feedstock is free for energy purposes, this brings significant energy system cost reductions, but when the energy sector pays for the biogas feedstock, then savings are lower, in which case biogas and biomethane still reduce the energy system costs for use in power, heat or industrial sectors. Replacement of liquid bio-electrofuels for transport with biomethane shows slight cost reductions, but considerably higher costs when using electromethane. For power, heat, industry and partly transport, electromethane is economically unfeasible, independent of the dry biomass costs. Biogas should be used directly or in the form of biomethane. It is a limited resource dependent on the structure of the agricultural sector, but it can supplement other renewable energy sources.