APOLLO SGP-210 series is stand-alone bidirectional inverter that is specially designed for solar backup system where AC power output is needed. It can be incorporated with the utility power line or a diesel generator or the battery bank with an external charger system which is used as the priority energy source. The external charger systems are the set of solar array with charge controller, the set of wind turbine with charge controller, the set of Pico hydrogenerator with charge controller or others. In general, the inverter supplies energy from the battery bank of the external charger system until the battery voltage reach the battery level then switch to supply energy from the utility power line or diesel generator. This stand-alone inverter controlled the operation by microprocessor, supplies pure sine wave voltage with low harmonic distortion and indicates the operating status of inverter to user with LED indicators.