This paper presents a study of mismatch losses when solar cells are associated in series to form photovoltaic (PV) modules. In this experiment, more than 10,000 single-crystal and multi-crystal cells were measured and sorted according to cell maximum power (P-mpc) and short circuit current (I-sc). Afterwards, PV modules were manufactured and measured with cells from each group. A selection of single-crystal cells with an efficiency rate of 17.75% was studied with results showing low relative power loss (RPL) rates. This was the case even when artificially increasing the scattering of mentioned electrical parameters. Due to these results, a second choice was made to perform a similar comparative study of multi-crystal cells. In the second study, the multi-crystal cells showed a greater scattering in groups of a 14.57% efficiency rate and of a 14.23% rate. In that case, results obtained for both groups showed a relation between RPL and standard deviation of electrical parameters. Despite there already being a widely recognized definition of RPL, due to the increased volume of cells in production environments, which is where this study was performed, the need was found for more effective and practical expression of RPL due to the large quantity of samples. Consequently, this paper proposes an innovative statistical equation of RPL relevant for the growing needs in this sector which can handle the parameters usually available to a module manufacturer. This equation is theoretically validated by comparison with Bucciarellis results (1979). The proposed expression of RPL has been applied to this study and an empirical relation between RPL and standard deviation of solar cell parameters has been obtained. (C) 2013 Elsevier Ltd. All rights reserved.