A power supply apparatus for supplying power to an external electrical load, the power supply apparatus comprising: a power transmission line or cable through which an alternating or direct current may flow; a power supply module including an input terminal connected to the power transmission line or cable so that the power supply module is connected in series with the power transmission line or cable, the power supply module including a plurality of switching elements (T 1 D 1, T 2 D 2) and at least one energy storage device, the or each energy storage device configured to be capable of storing energy to provide a voltage thereacross and releasing energy, the plurality of switching elements (T 1 D 1, T 2 D 2) being arranged to permit switching of the or each energy storage device into and out of circuit with the input terminal; a control unit programmed to control the plurality of switching elements (T 1 D 1, T 2 D 2) to selectively switch the or each energy storage device into circuit with the input terminal to direct a current flowing in the power transmission line or cable to flow through the or each energy storage device so as to store energy in the or each energy storage device to form a power source; an output terminal for connection to the external electrical load; and a converter connected to the power supply module to operably couple the or each energy storage device to the output terminal, the converter configured to draw power from the power source and supply the drawn power to the output terminal; wherein the converter is configured to regulate the power supplied to the output terminal so as to maintain the voltage across the or each energy storage device within a target range. a power transmission line or cable through which an alternating or direct current may flow; a power supply module including an input terminal connected to the power transmission line or cable so that the power supply module is connected in series with the power transmission line or cable, the power supply module including a plurality of switching elements (T 1 D 1, T 2 D 2) and at least one energy storage device, the or each energy storage device configured to be capable of storing energy to provide a voltage thereacross and releasing energy, the plurality of switching elements (T 1 D 1, T 2 D 2) being arranged to permit switching of the or each energy storage device into and out of circuit with the input terminal; a control unit programmed to control the plurality of switching elements (T 1 D 1, T 2 D 2) to selectively switch the or each energy storage device into circuit with the input terminal to direct a current flowing in the power transmission line or cable to flow through the or each energy storage device so as to store energy in the or each energy storage device to form a power source; an output terminal for connection to the external electrical load; and a converter connected to the power supply module to operably couple the or each energy storage device to the output terminal, the converter configured to draw power from the power source and supply the drawn power to the output terminal; wherein the converter is configured to regulate the power supplied to the output terminal so as to maintain the voltage across the or each energy storage device within a target range.