One of the basic principles of the Energy Strategy of the Republic Croatia is to encourage the production and use of biogas as a renewable energy source to produce electricity and for heating purposes. The reason for this is certainly a need for the disposal of waste from agricultural production and the reduction of greenhouse gas emissions. This work is based on real investment of company engaged in the production of milk in renewable energy sources and to the project of building biogas where it will be produced electricity and heat, and as a raw material it will be used by-products from the company's core business. Analysis of investment in the construction of this plant was carried out with the technological and financial aspects. As a part of the technical and technological aspects it was analyzed the impact of the biogas plant to the quality of the voltage in the system, in the part of economic and financial analysis it was explained a detailed analysis of revenues, the economic justification - profitability, liquidity analysis and market assessment of the project in view of the return on investment investment, used from different methods that are estimating the cost-effectiveness