A simple computer simulation model suitable for the design of large solar water-heating systems has been developed. Compared with existing simplified correlation design methods, the present model allows a remarkable expansion of their applicability so as to incorporate any hourly demand profiles, solar system design configuration and heat losses from various system components, and remove any imposed restrictions referring to the definition range of the large number of design parameters involved. This flexibility is offered at no extra cost of complexity, long user experience and familiarity with the use of a complex computer code, something which is particularly important, especially for the practising solar designer or field engineer. Numerous results are presented referring to the effects of the variation of a broad range of main design parameters on system performance. The accuracy of the present model is validated against corresponding results from the familiar f-chart design method, the accuracy of which has been found to be very good according to comparisons between calculations and measured field data, as well as TRNSYS simulation results.