In this paper the term of reactive power compensation and benefits of investment in the application of the system of compensation are considered. First of all, the theoretical basis and a clarification about the power, active and reactive is given. Definition and description of reactive power compensation, a way of determining the power necessary compensations, as well as a way to compensate reactive power through all types is given. Devices that participate in the process, as analyzers and as expansion joints are described. The theoretical foundation of the concept of Cost-benefit analysis is given. An instrument named Metrel PowerQ4 is used for the practical part of the paper. Clear and detailed steps for the preparation and shooting and using the instrument in exploitation are presented as well.