An uninterruptible power supply for supplying power to a load, the uninterruptible power supply comprising: an input section adapted to be connected to a primary power source; an output section adapted to be connected to the load; an inverter section adapted to be connected to a secondary power source; and a ferroresonant transformer comprising a core, a first shunt arranged relative to the core to define a primary side and a secondary side of the ferroresonant transformer, first windings arranged on the primary side of the ferroresonant transformer, where the first windings are adapted to be connected to the input section, second windings arranged on the secondary side of the ferroresonant transformer, where the second windings are connected to the inverter section, and third windings arranged on the secondary side of the ferroresonant transformer, where the third windings are connected to the output section; characterised in that when a primary signal from the primary power source is present at the input section, the output section may be configured to connect a resonant capacitor to the third windings to form a resonant circuit and a first output signal is present on at least a portion of the third windings; and when a secondary power signal from the inverter is present at the inverter section, the output section may be configured to disconnect the resonant capacitor from the third windings and a second output signal is present on at least a portion of the third windings. an input section adapted to be connected to a primary power source; an output section adapted to be connected to the load; an inverter section adapted to be connected to a secondary power source; and a ferroresonant transformer comprising a core, a first shunt arranged relative to the core to define a primary side and a secondary side of the ferroresonant transformer, first windings arranged on the primary side of the ferroresonant transformer, where the first windings are adapted to be connected to the input section, second windings arranged on the secondary side of the ferroresonant transformer, where the second windings are connected to the inverter section, and third windings arranged on the secondary side of the ferroresonant transformer, where the third windings are connected to the output section; characterised in that when a primary signal from the primary power source is present at the input section, the output section may be configured to connect a resonant capacitor to the third windings to form a resonant circuit and a first output signal is present on at least a portion of the third windings; and when a secondary power signal from the inverter is present at the inverter section, the output section may be configured to disconnect the resonant capacitor from the third windings and a second output signal is present on at least a portion of the third windings. a core, a first shunt arranged relative to the core to define a primary side and a secondary side of the ferroresonant transformer, first windings arranged on the primary side of the ferroresonant transformer, where the first windings are adapted to be connected to the input section, second windings arranged on the secondary side of the ferroresonant transformer, where the second windings are connected to the inverter section, and third windings arranged on the secondary side of the ferroresonant transformer, where the third windings are connected to the output section; characterised in that when a primary signal from the primary power source is present at the input section, the output section may be configured to connect a resonant capacitor to the third windings to form a resonant circuit and a first output signal is present on at least a portion of the third windings; and when a secondary power signal from the inverter is present at the inverter section, the output section may be configured to disconnect the resonant capacitor from the third windings and a second output signal is present on at least a portion of the third windings.