Direct current (DC) motors and centrifugal pumps operate at variable speed in a photovoltaic pumping system. However, the characteristics of motors and pumps are usually only given for a single voltage and speed. To analyse the diurnal operation of a pumping system, where the solar radiation varies causing variation in the photovoltaic power, the input/output relationship of the motor pump assembly must be determined. In this article we show that pump similarity laws may be applied to the analysis of centrifugal pump performance for variable speed operation. Modelling equations of the motor pump assembly are derived from single speed pump characteristics and a no-load motor test. The application of the modelling equations in conjunction with the equation characterising the photovoltaic array yields the relationship between solar radiation and pump flow rate for a given pipeline.