PhD thesis addresses the calibration of power and effective AC measurement standards for the measurement of power and RMS value of AC signals. Microcalorimeter represents an instrument for effective efficiency measurements of test standard by measuring losses due to heat transfer. Design of the microcalorimeter is based on modelling: double shielded housing; isolation section (heat transfer optimization); coaxial transmission line; elements for detection of losses in bolometer housing; DC components for the realization of bridge circuit in order to hold bolometer constant resistance; A correction factor of microcalorimeter is determined by measuring heat losses and heat effects (losses in connectors, in bolometer mount). The main purpose of PhD dissertation was to develop a microcalorimeter structure for accurate power measurement in the frequency range of 500 MHz - 6 GHz intended for use in smart grids. The first chapter (1. Introduction) summarizes the motivation and goals of the doctoral thesis and elaborates the content of the of dissertation. Also, individual parts of work that preceded the final realization of microcalorimetric measurements of AC signal power based on a selfbalancing bridge are. The second chapter (2. Microcalorimeters and transfer standards) summarizes the development of microcalorimeters over the decades with special emphasis on the components of which the microcalorimeter consists of. Also, a review of the latest research in the field and a chronological path to the appearance of microcalorimeters with an interruption in the outer conductor of the adiabatic line is given. The third chapter (3. Smart Grid communication and energy harvesting) introduces Smart Grid communication and energy harvesting systems, where exists the application of microcalorimeters, with special reference to existing wireless communication networks and frequency bands. v In the fourth chapter (4. Definition of electrical power) the definition of electrical power is given according to the standard IEEE Std-2010. This standard describes the components of power and theoretical basis of power measurement. The fifth chapter (5. Precise measurement of power and RMS value of the AC signal) summarizes the measurement procedure for precise power measurement using thermal power detectors with special emphasis on the calibration procedure. Thermal converters and their possibilities for measuring power at high frequencies are also briefly described. Chapter 6 (6. Microcalorimetric Power Measurement at High Frequencies) describes the procedure for measuring the power at high frequencies using a microcalorimeter. This chapter is the most important chapter of the doctoral thesis because it presents: simulation results of the adiabatic line, a prototype of the developed device and electrical and thermal characterization of the adiabatic line. Also, thermal detector is described which is used in prototype as well as a double metal thermostat housing. In addition to the above, a description of the thermal characterization and temperature regulation of the thermostat. In the seventh Chapter (7. Balancing of the bridge circuit) describes the process of balancing the bridge connection and the acquisition of measurement data by digital multimeters with a resolution of 8 ½ digital with synchronization and control through the LabVIEW system. The eight chapter are conclusions.