Abstract: A modular power supply (2) suitable for application as an uninterruptable power supply for use with electrical equipment such as computers. A module (6) of the power supply may include a battery (32), a charging circuit for the battery (34), a power supply circuit coupled to mains electricity and the battery (30, 36, 40), and a control circuit (38) for selectively providing power to the electrical equipment (12) by way of the power supply circuit from either mains electricity (10) or the battery as source. The module is also provided with a coupling which is adapted to connect the module in parallel with at least one other module. A housing may be provided to receive a plurality of modules, which are connected together in parallel by way of plugs and sockets when received in the housing. The plugs and sockets and the control circuit may be adapted to enable "hot-swapping" of the modules from the housing.