PV module power ratings are determined at standard test conditions, which require perpendicular incident light. Under field conditions larger incidence angles occur, resulting in higher reflection losses than accounted for in the nominal power rating. In this article we will present a model to take these losses into account, and discuss some results for practical situations. From our model we conclude that the reflection losses relative to STC are determined mainly by the air-glass interface. Limited validation assuming certain spectral losses showed a rough correspondence between calculated reflection losses and experimental values on a yearly averaged basis (1.2% difference between model and experiment). Model calculations show that for modules faced towards the equator, and with a tilt angle equal to the latitude, yearly reflection losses relative to STC are about 3%. For this tilt and orientation, the losses seem to be only slightly dependent on the geographical latitude of the location. Tilt, orientation and seasonal dependence are significant. For vertically mounted PV modules (facades) near the equator the reflection losses can be quite large (up to 8%).