We determine the threshold herd size at which a biomass waste to energy conversion system becomes commercially viable. The threshold herd size is found by optimisation of the biomass waste to energy conversion system for different herd sizes. Manure is co-digested with food waste. The optimisation takes into consideration revenue from electricity sales, use of the by-products of the digestion and food waste tipping fees. The cost implication of cleaning the biogas combusted in the internal combustion engine is also discussed. The threshold herd sizes for two different types of farms is determined.