A prototype photovoltaic-powered reverse-osmosis system has been constructed at CREST, Loughborough. UK. The rate of production of fresh water varies throughout the day according to the available solar power, and thus, the system operates without need of batteries. The system is designed to operate from seawater, and a Clark pump brine-stream energy recovery mechanism is coupled with a variable recovery ratio technique to achieve a specific energy consumption of less than 4 kWh/m(3) over a wide range of operation. Measurements showing the variable operation over a two-day period are presented and discussed.