ln the present paper a method to evaluate the room temperature effect in deteriorating the performance of Silicon flat plate PV systems is investigated. The calculations, applied to crystalline Silicon arrays, show a significant energy loss with respect to reference temperature conditions, specially in correspondence to the sites with highest insolation, in which the effect can amount up to about 15%. As a consequence, the previous projections about kWh cost for large scale PV power stations should be rectified by introducing a correction factor accounting for thermal effects. The calculations were carried out for 64 zones, particularly suitable for siting PV plants and distributed among 15 Countries over a latitude belt ranging from about 40 North to 15 South. For each zone, it was accounted both for room temperature and insolation historical data. The final results show a linear correlation between yearly global sun radiation density and room temperature effect, which appears quite general, so that, for any other site within the latitude belt, it is possible to account for thermal effect on PV kWh cost simply by knowing the global sun radiation density on horizontal surface (usually the only experimental available datum).