In this paper, the methods of mathematical modeling are used to investigate an autonomous hybrid energy system with renewable generation. The aim of the study is to determine energy system optimal structure and its economic efficiency. The energy system contains photovoltaic modules, solar collectors for the solar heating system, storage batteries, a diesel power plant, a pellet-fueled boiler house, a biomass gasification mini-CHP operating on wood chips. A biomass gasification mini-CHP is used to cover the solar energy sources generation deficit, as well as to effectively replace expensive diesel generators. The paper considers two variants of mini-CHPs, differing in the cost of the gasifier. The mathematical model REM-2 (Renewable Energy Model) applied for calculations does not require an analysis of the structure options and the preliminary specification of the algorithm for switching energy flows between energy sources, consumers, and the battery. Optimal operating conditions are determined on the basis of GAMS (General Algebraic Modeling System) algorithms, taking into account the random nature of solar energy and the variability of energy consumption. The optimal structure of the system (installed capacity of energy sources and battery capacity) is determined, dynamics of energy production in summer and winter periods is investigated. Mathematical modeling that accounts for seasonal factors has demonstrated the efficiency of the joint use of solar energy and wood biomass.