The long-standing question of the most appropriate dimensionless groups to be used for the correlation of the performance of solar domestic hot water (SDHW) systems is reviewed. It is shown that first principles enable two groups to be defined which collapse both the correlations of Kenna and f-chart to a single curve, for a given ratio R of the daily demand to storage mass. These are the groups that determine the initial gradient of the curve and its final asymptote. Using a thermal model of the collector-store combination, it is then shown how the shape of the remainder of the correlation curve also derives from basic physical principles. This model provides a convenient instrument for illustrating the sensitivity of system performance to the many factors that affect it. Some, such as the shape of the irradiance pattern at the collector, are found to be relatively weak. It is further shown why systems with differing values of R cannot be represented by a single correlation curve. However, a set of coordinates are suggested which nearly collapse the curves for variable R, providing a convenient format for presenting correlations of simulated or measured data for use in long-term performance prediction and the checking of performance claims.