Abstract: A power supply has a plurality of single line interface circuits (20, 22, 24 and 26) each connected by a respective line, to a respective telephone (4, 6, 8 and 10) or other telecommunications device. Each SLIC applies a sufficient voltage derived from a battery to the line to operate the respective telephone, and dissipate any excess power supplied by the battery. The apparatus includes common power regulation means (30), which incorporates a switch mode power supply unit (68), and which monitors the voltages being supplied by the SLICs at any one time and reduces the voltages applied to the SLICs to substantially the maximum voltage required by any one of the SLICs, thereby reducing the amount of power dissipated by the SLICs.