The paper aims to analyze the energy and economic effect of a demand-driven biogas investment versus a conventional one in modern dairy farm facilities of medium scale. Demand-driven exploitation may be the only way to obtain permission for connection to energy grid of new renewable energy sources (RES) installations in administrative regions where intermittent RES i.e. photovoltaics provide a large amount of energy to the grid. The biogas plant cogeneration capacity is meant to respond to demand out of the time range of intense electricity generation from photovoltaic farms. The energetic and economic analysis showed that the investment in the biogas plant would be profitable (payback period of 3.41 years in the best scenario), achieved thanks to operating as demand-driven biogas plant with CHP 499 kWe working 12 h/day (4250 h/year). Since the biogas plant is planned to operate as a demand-driven installation, it would also be possible to perform service inspections of the cogeneration unit (CHP) when it is not working. As a result, this small biogas plant (250 kW) with 499 kW CHP can produce as much as 2126 MWh/year. The second aspect of profitability is the use of biomass (mainly biowaste) from a farm as a substrate.