A general method is described for calculating the current drawn by an inverter from its DC source. It has special relevance to pulse-width modulated inverters and allows the ripple current rating of the filter capacitor to be specified with some precision. The method is for an inverter of any number of phases and uses switching functions derived from the harmonic coefficients of the inverter waveform. Experimental results from single- and 3-phase inverters demonstrate the validity of the method. Some generalised results are presented for a 3-phase inverter for the special case where the output current can be assumed to be the fundamental component only. Variation of the DC link current harmonics with power factor of the load and depth of modulation under these conditions is presented and discussed.