The paper proposes the application of PV system for uninterruptible power supply (UPS), The PV system is composed of a novel single-phase PWM voltage source inverter (VSI). Composite PWM method is achieved by a circuit configuration consisting of the normal single-phase bridge circuit and an additional arm, which contributes to reduce the ripple in the AC output current. The VSI has a LC series-resonance circuit tuned to twice the utility frequency connected in parallel with the smoothing capacitor, The series-resonance circuit absorbs the double-frequency AC components included in the DC pulsated current. Hence, the smoothing capacitor is drastically reduced. A storage battery is added on the DC side in order to stabilize DC voltage. The VSI is easily changed from inverter action to rectifier action by changing the firing angle alpha from plus to minus. When alpha is minus, the VSI acts as a battery charger.