3 sets of PV modules, each set having Si cells of a different morphological structure (a-, me- and pc- ), were operated side-by-side as grid-connected systems, and continuously monitored for one year. The resulting hourly data were re-normalized in a number of alternative ways so as to permit meaningful efficiency comparisons. In particular, comparisons were made with respect to (1) Total system efficiency; (2) Specific energy delivery [Wh/Wp(STC)]. The purpose of the study was to identify external conditions under which the relative rating of any pair of module types can become interchanged. We find that although the a-Si system gave a higher specific energy delivery throughout the summer half of the year, on an all-year basis, the c-Si modules produced slightly higher overall figures. The slight summer advantage of a-Si is attributed to a combination of spectrum and temperature effects.