Both for photovoltaic and photovoltaic/thermal applications insight is required in the mechanisms that determine the effective absorption factor Aeff. Aeff is the part of the incident irradiation that is converted into heat, taking into account that part of the energy is withdrawn as electricity. Aeff was studied for five different solar cell technologies using an optical simulation model and ranges from 74% for single junction amorphous silicon solar cells to 82% for CIGS solar cells. The simulations also show that the longer wavelength part of the spectrum is hardly absorbed by the active semiconductors, but mostly by free carrier absorption in the transparent conductive oxide film present in these devices.@en