The software uses the equations (1) to build the concentrator and the equations (2) — (19) to calculate the radiant flow density on the input aperture and the optical concentration factor [16, 17, 18, 19]. We consider that the radiation that is incident on the photovoltaic cell is fully absorbed by it. The radiant flow density on the output aperture, Brec, is calculated with the equation:

The meanings of the measures in the (20) equation are: Bconc — the radiant flow density on the

input aperture, R — the input aperture radius; r — the output aperture radius; Ninitial — the initial numbers of rays; Nabs — the number of rays on the output aperture considered to be absorbed of photovoltaic cell. The measures given by the user are: the input aperture radius, R (mm); the output aperture radius r(mm) considered to be equal to the receptive cell radius; the position of the receptive cell, H0; the initial moment of the measures, t0; the final moment of the the measurings, t1; the incrementation of the time At; the inclination angle of the roof, s; the month of the year, l; the day of the month, %.

The calculated and shown measures are: the intensity of the solar, Bn; the angle of incidence of the radiation on the input aperture, в; the density of the radiant flow in the plane that contains the input aperture, Bconc; the density of the radiant flow in the plane that contains the photovoltaic cell Brec; the optical concentration factor, Coptic; the quantity of energy that passes the input aperture during the measurements, Qconc; the quantity of energy received by the cell, Qrec ; the concentrator’s efficiency q.

The concentration is efficient if the optical concentration factor is bigger than 1. With the help of the tables or of the graphics we can determine the maximum angle of incidence 6max and the time period for which 6<Bmax.