The purpose of this work is to develop the fundamentals of a methodology for evaluating the efficiency of solar thermal systems. Research Methodology: The study is based on general scientific methodology, which implies a systematic approach to problem-solving. Results: The article presents a methodology for assessing the efficiency of solar thermal systems used for heating and hot water supply in residential, agricultural, and industrial buildings and facilities. These systems are primarily used in regions with favorable climatic conditions. Azerbaijan is one of the most suitable countries for solar energy utilization. The amount of solar energy incident on a horizontal surface throughout the year varies approximately from 1500 to 2000 kWh per m². The annual duration of solar radiation ranges from 2000 to 3000 hours. This value is high compared to other countries due to the country's geographical location, the number of sunny hours, and other natural conditions, indicating broad potential for solar energy use. Solar thermal systems are competitive and can provide positive economic, social, and environmental effects. Application Area: Design and implementation of solar energy systems, systems combined with solar collectors, heat pumps, and thermal accumulators, as well as simple water heating devices intended for supplying hot water and heating for residential houses and other consumers requiring low-temperature heat. The development of these technologies is crucial for the gradual transition to houses fully supplied with solar thermal energy. Conclusion: Precise calculations and tests conducted in Azerbaijan show that these solar systems have high energy and economic efficiency.