PV modules which are less sensitive to temperature (i.e. have lower power temperature coefficient, PTC) are expected to produce more energy in hot environments. Yet a year-long study of 26 PV technologies in Doha, Qatar, found that their daily Specific Yield (Wh/day/W-p) only weakly correlated with their PTC. To investigate possible explanations, we examined in detail the field data from one multi-Si and one CdTe string, and calculated their energy losses attributable to module temperature, low-light conditions, and "W-p discrepancy" (difference in W-p between manufacturers' flash tests and field measurements). We found that although the CdTe system had lower PTC than multi-Si, it had larger W-p discrepancy, which largely accounted for its lower Specific Yield. Further, PTC values derived from field measurements were significantly different from the modules' nameplate values, for both the multi-Si and CdTe systems. The results indicate that modules' nameplate PTC values did not strongly predict their relative Specific Yields in desert climates, largely because of discrepancy between nominal and field values of PTC and W-p.