Abstract: The invention will be applied in industry and households. The method includes performance of an electrolysis process. A first heat-transferring fluid is heated directly in the electrolytic cell (2) by the heated electrolyte (3). A second heat-transferring fluid is heated with the released oxy-hydrogen gas (4) by a gas flame burner (5). Both heat-transferring fluids—through independent from each other circulation circuits (6, 7)—give their heat in an accumulating vessel (8) which contains the liquid (1) to be heated. The method and the device for its realization allow heating the liquid within a short period of time and with low energy consumption. Small-sized devices are designed according to the invention, with high efficiency and safety in use and no harmful environmental emissions are released. The direct use of energy from alternative sources is possible.