An accurate method of determining the maximum power point (V_m, I_m) of an energy conversion device like a solar cell, based on simple geometrical considerations, is given. Two additional checks on the correctness of the point are also possible by the method. The procedure holds irrespective of the values of parameters like the diode ideality factor (A), the series resistance (R_s) and the shunt resistance (R_sh), and enables an accurate evaluation of the fill factor (FF). The reliability of the method is established using the V against I curves for an Si homojunction, a Cu₂S/CdS heterojunction and Al/Si, MIS Schottky barrier cells. It is shown that possibilities exist of quite accurate, consistent and comprehensive measurement of the light generated current I_L, A and R_s if the effect of R_sh is negligible, and the reverse saturation current I_o determined from the characteristics without illumination. Typical examples of the application of the method are given. It is found that the value of R_s, determined by the method, is always larger than that obtained from the slope of the V against I curve near the V-axis. This is shown to be expected on physical grounds. However, on the same basis, it is found that the value of R_sh, as obtained from the slope of the reverse characteristic under illumination, should not differ much from the value at the operating point, i.e. (V_m, I_m)