There are methods of supplying the required energy to the load while reducing the cost relatively, but do not take into account the amount of reserved energy; this is not preferred if batteries are not used to store the excess energy produced. The proposed method is a mathematical model to calculate the system size for supplying a load in an area far from the electrical grid where there are huge sources of biomass energy and high rates of solar radiation. The system includes three energy sources to generate electrical energy from solar energy, biogas in addition to a diesel generator supplying a specific load without batteries. The proposed model provides a solution that achieves an optimized system design while minimizing reserved energy taking into account minimizing the cost. The method assumes that the load is supplied first from the solar generator, when the load gets higher, the biogas generator is introduced, and the diesel generator works only if the load exceeds the capacity of the renewable sources. This scenario depends on increasing the contribution of renewable resources compared to fossil fuel.