Because the AC•THOR is an alternating current (AC) device. In an off-grid system, it requires a PV inverter and a battery inverter that performs frequency modulation depending on the battery charge status. As the frequency increases, the AC•THOR increases the output of heat. In off-grid systems without frequency control, the power control of the AC•THOR must be done differently, for example, through a higher-level control system, such as an energy management system.