An uninterruptible power supply (UPS) system (100) comprises a plurality of UPS units (UPS-1, UPS-2) connected in parallel. The controllers (130) of the units are programmed to implement a voltage calibration procedure and a current calibration procedure, in order that measurements of voltage and current made by sensors within the different units will agree. In the current calibration procedure, the load is disconnected (302) while one of the units is selected as a master and operates in a voltage control mode (VCM) (Steps 304-308). Each other unit is selected in turn and operated in a current control mode (310, 312). Current measurements made in the master unit are communicated (314) via a data bus to the selected unit and compared (316) with measurements made in the unit itself. The unit adapts its current sensing gains to match the master unit.