Abstract Building-integrated solar thermal systems (BIST) outperform building-added solar thermal systems (BAST) due to smaller heat losses at the back of the collector. BIST offer economic advantages, too. The insulation behind the collector can be used to reduce the heating demand of the building as well as to increase the solar thermal yield. Therefore, less material and labour are needed. Of course, the energy flux to the building interior needs to be considered. This energy flux depends in general on the operation of the collector as well as on the irradiance. Several innovative solar thermal building skins have been modelled in detail to analyze this coupling between the active building skin and the building. However, planners need an easy approach to include BIST into their calculations. Often, there is not enough budget to measure and model the new facade. This paper presents several new and simple models which are more accurate than neglecting the coupling to the building and which are less complex than detailed physical models.