At least some aspects of the invention are directed to methods and apparatus for controlling an uninterruptible power supply and subsystems of a UPS. A first aspect of the invention is directed to a method of controlling a DC-DC converter having a predetermined maximum peak load current value. The DC-DC converter has first and second outputs to couple to a load with a capacitor coupled across the first and second outputs. The method includes in a first mode of operation, charging the capacitor to a predetermined output voltage value, and in a second mode of operation, providing output current having the maximum peak load current value to a load coupled to the output of the DC-DC converter, wherein a first portion of the output current is provided by the DC-DC converter and a second portion of the output current is provided by discharging the capacitor to a voltage value that is less than the predetermined output voltage value.