An uninterruptable power supply includes a rechargeable battery, having a positive terminal and a negative terminal, and a transformer. The transformer includes a battery side transformer winding having a first terminal, a second terminal and a third terminal; a load side transformer winding inductively coupled to the battery side transformer winding via a core; and a power supply side transformer winding that is inductively coupled to the battery side transformer winding via the core. An inverter bridge circuit couples the positive terminal of the battery to the first terminal of the battery side transformer winding and couples the negative terminal of the battery to the second terminal and the third terminal of the battery side transformer winding. The inverter bridge circuit includes a switching circuit that forms a bridge circuit that allows current from the battery side transformer winding to flow in only one direction into the battery when the switching circuit is in a first switching state so that current from the power supply slide transformer winding induces an alternating current in the battery side transformer winding that passes through the bridge circuit to generate a direct current that charges the battery when the switching circuit is in the first switching state. The switching circuit also forms an inverter circuit that generates an alternating current from a direct current supplied by the battery when the switching circuit is in a second switching state, thereby inducing an alternating current in the load side transformer winding. A control circuit drives the switching circuit into the first switching state when at least a predetermined power level is supplied to the power side transformer winding. The control circuit also drives the switching circuit into the second switching state when less than the predetermined power level is supplied to the power side transformer winding.