The optimal orientation of a collector (e. g. south, tilted according to the latitude) is often not to choose. To take into account the local conditions (roof slope, roof orientation, site orientation) it often needs a compromise. CPC vacuum tube collectors have the advantage that they can compensate an unfavourable orientation with the smallest additional collector area. In an extreme case with east orientation flat plate collectors need 90 % more area. With vacuum tube collectors this would be only 40 %. This is because of the better Incidence Angle Modifier (IAM) and the higher efficiency factors with small irradiation as usual with unfavourable roof orientations as well as in the morning and evening.