When considering the design of a PV/thermal system, determination of the ratio of the values of the electrical and thermal output from the system allows a rational approach to design optimisation via the minimization of 'equivalent electrical levelised energy cost'. This paper focuses on methods that can be employed to develop a ratio between electrical and thermal output from a domestic style PV/thermal system. Methods discussed include thermodynamic analysis using exergy; market analysis for both an open market and a renewable energy market; and environmental analysis using avoided greenhouse gas emissions. Ratios are developed for each method based on real data. It is concluded that a renewable energy market approach seems most logical for such a system, and an indicative value of 4.24 is obtained. An example is given comparing a PV/thermal system that uses amorphous silicon cells with one that uses crystalline silicon cells. Levelised energy cost is plotted against the energy value ratio to show that there is a critical electrical-to-thermal energy value ratio below which a collector with a-Si cells is more cost effective than one with c-Si cells. (C) 2003 Elsevier Ltd. All rights reserved.