An integrated collector/storage solar water heater (ICSSWH) that can significantly reduce heat loss to ambient during non-collection periods has been developed. Two thirds of the ICS vessel is mounted within a concentrating cusp, McIntire 'W' modified concentrator and incorporates an inner heat retaining vessel. The remaining upper 1/3 of the vessel is situated outside the reflector cavity and is heavily insulated. Over 60% of the thermal energy stored within the total vessel, and up to 67% of that in the upper immediate draw-off region can be retained over a 16-h non-collection period. Results of an experimental analysis of this design and a comparison with a standard ICS design are presented. (C) 2003 Elsevier Ltd. All rights reserved.