One of the main tasks arising in power supply systems (PSS) of spacecraft (SC) design is the determination of rational structure in terms of providing consumers with electricity of the required quality. At the same time, a reasonable consumption of power generated by solar batteries (SB) and accumulator batteries (AB) should be realized in PSS. The choice of the PSS structure is based on the calculation and comparative analysis of PSS options, taking into account the adopted system performance criteria, the main ones being the energy and weight-dimension characteristics. For this purpose, the process of energy flows distribution in the PSS by forming a mathematical description of the PSS operating modes is carried out. In order to obtain the graphs of the SB generated power and to calculate SB parameters during the service life, a mathematical model of the SB based on the use of initial and experimental parameters of its photovoltaic elements of any area was developed. The SB model provides the required accuracy of I-V and V-W characteristics calculation for any given values of illumination and temperature.In the article the method for calculating the energy characteristics of PSS and SB parameters taking into account the possibility of its limitation at the maximum or minimum level was described. It is shown that the method allows to determine the ways of rational redistribution of energy flows in the systems being designed to improve its weightdimension characteristics by reducing the maximum design power of energy-converting equipment (ECE), which is achieved by forming a rational logic for applying the SB maximum power point tracking mode, in particular, when the spacecraft leaves the Earth's shadow. Energy balance in PSS is provided by applying correction coefficients. The calculation results obtained by the method are the basis for requirements for ECE and SB design in PSS and can be used by developers and manufacturers of onboard and ground PSS.