A description of the developed models, algorithms and software for operational assessment of the energy balance on board spacecraft for optoelectronic observation of the Earth’s surface in order to monitor the achievement of the performance targets of a space observation system is given. On the basis of the obtained models and algorithms, a software module was developed for assessing the onboard energy balance in real time, used as part of the software for assessing the target and main design indicators of the space surveillance system. The module’s user interface provides windows for entering and controlling the initial data according to the parameters of the power supply system and cyclograms of connecting on-board system devices, as well as windows for displaying the results of energy balance modeling in digital form and in the form of dynamic diagrams. The use of software based on the proposed models and algorithms makes it possible to more reasonably and quickly develop the initial requirements for the designed onboard systems of remote sensing spacecraft of high-detail and operational observation and to accelerate the process of coordinating design characteristics of the spacecraft at the initial stages of design. The proposed methods, models and software are universal and can be used in the design of Earth remote sensing spacecraft with a wide range of target indicators, target equipment composition and onboard support systems.