Solar cells are not only environmentally friendly, but save considerable electricity costs as well (namely, in this case, only maintenance costs have to be paid, the value of which is significantly lower than the price of electricity). The long term aim is for these energy sources to be applied and used by all macroeconomic actors (companies, households, the government and other institutions). Such cells can be introduced by applying for a considerable amount of investment subsidies at an EU level and in Hungary, as well. However, the ROI of this kind of undertaking is long term. The aim of this study is to examine whether a non-profit public institution would find it worthwhile to invest in this type of venture in the long term. Thus, the ROI of a solar cell investment was examined at a well-known environmentally public institution, at an abbey in Hungary. Data were provided by the abbey. From available data, ROI calculations were carried out and the approximate payback period was estimated. Calculations were carried out taking into account different scenarios. One part of the research focused on the inflation rate (there was a case where the inflation rate was ignored), in the other part, the cost of investment was taking into account in different ways. The payback period of solar cell investment is relatively short (11-13 years) in the case of EU or government subsidies, otherwise it is quite long (25-30 years).