They do it by combining surge capacity, fast control response, battery support, and protective circuit design. In a stand-alone system, the inverter acts as the voltage source for the load, so it must hold voltage and frequency steady even when motors, pumps, compressors, or other equipment create a sudden startup demand. U.S. Department of Energy material on distributed PV design notes that a stand-alone inverter must supply current surges demanded by motor startup and the reactive power required by the load while maintaining voltage quality.