The book brochure discusses the measurement of active and reactive power in electrical installations of industrial and high frequency. Schematic diagrams and vector diagrams with brief theoretical justifications are given. Practical schemes for measuring power under normal operating conditions are described in detail, and the features of measurements during testing of generators, transformers, power lines and electric motors are emphasized. Information on power measurement of household electrical appliances is given. Typical cases of measurements are illustrated by examples for the selection of instruments, diagrams and evaluation of measurement results. The book is intended for electricians who have completed secondary education, are familiar with the principle of operation and design of electrical measuring instruments and take part in tests of electrical equipment. It can be useful to the average technical staff and students.