The paper addresses an analysis of the efficiency and profitability of the operation of a photovoltaic installation located in the geometric centre of Europe (near Białystok, Poland), where the intensity of solar irradiation is not too high compared to other European countries. It is calculated that in that place average solar irradiation being lower even by approx. 26 kWh than that for the whole Europe, which results in a 26% drop in the economic potential of the utilisation of solar energy for its conversion. A case study and an economic analysis show that without minimum funding amounting to 50% of the investment costs paid for the modernisation of a central heating system assisted by PV cells, the time of return of pecuniary expenditures exceeds 7 years. Apart from the Simple Pay-Back Time SPBT, discount indicators determined in the paper also include the net present value NPV and the internal rate of return IRR. Moreover, a direct ecological effect has been determined for such an investment.