The variation of the optimum efficiency of an array of silicon solar cells with diffuse ratio has been investigated experimentally. Data collected from two geographical locations in Nigeria (Port Harcourt, which is on latitude 4.43°N and longitude 7.10°E; and Enugu, which is on latitude 6.20°N, and longitude 7.30°E), show that the optimum efficiency of an array of silicon solar cells increases with diffuse ratio. Under clear and partly overcast weather conditions, a change of 0.1 in the diffuse ratio results in 5 to 10% change in efficiency, while the same change under overcast weather conditions, results in 20 to 35% increase in optimum efficiency. Temperature fluctuations contribute less than 4,26% of the total change in efficiency.