Maximization of energy delivery is the fundamental problem in solar collector thermal design. This paper examines the trade-off between the storage and the immediate use of solar exergy, with the objective of maximising the long-term exergy output from a solar collector installation. It is demonstrated that a trade-off exists, and that for maximum exergy output the collector and the collector-user interaction must conform to a well-defined pattern. It is also shown that the practice of operating collectors at constant temperature, regardless of time of day, is responsible for a sizeable and steady exergy loss. © 1982.