We have studied the response of the efficiency and short-circuit current of an array of silicon solar cells to changes in intensity of solar radiation under tropical atmospheric conditions. The investigation was carried out simultaneously at two different geographical locations in Nigeria: Enugu (6.20°N, 7.30°E) and Port Harcourt (4.43°N, 7.10°E) (Port Harcourt is located very close to the sea, while Enugu is far away from the sea). Data collected from the two locations clearly reveal that the mean optimum efficiency is not constant, but decreases exponentially as the intensity of solar radiation increases. The short-circuit current, on the other hand, increases non-linearly with intensity of solar radiation and approaches some constant maximum value under very high insolation. The change in efficiency is above 70% as the intensity of solar radiation changes from 100 to 800 W m−2. An empirical equation, relating the mean optimum efficiency to intensity of solar radiation, and an empirical equation, relating the mean short-circuit current to intensity of solar radiation, have been proposed. No previous similar work is available for comparison, but our results are consistent with the well-established fact that under cloudy conditions (and thereafter low insolation), short-wavelength radiation becomes predominant.