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This study determined the power-generating capacity at which manure- and wastewater-to-energy conversion systems become commercially viable in Yunnan Province, China. Capacity was determined through technical and economic analyses of 12 full-scale biogas plants with 40-, 60-, or 80-kW engines. Each biogas plant made use of one of the two types of anaerobic digestion processes. Market and policy conditions, rather than technological limitations, resulted in the poor performance of both processes. Conversion systems with both normal-temperature and mesophilic digesters had relatively high wastewater treatment capacities of 0.06–0.10 m3 m−3 d−1, whereas systems with only one mesophilic digester had capacities of 0.03–0.06 m3 m−3 d−1. Capacity depended on demand by farms, not on the amount of fermentation material. Only the 80-kW biogas plants had acceptable payback times, together with positive net present values and internal rates of return. Furthermore, the price of digested effluent was an important factor that influenced the commercial viability of the 80-kW plants.