An exact explicit solution based on the Lambert W-function is presented to express the optimum load of an illuminated solar cell containing a parasitic series resistance and a shunt resistance. The W-function expressions are derived using Matlab software. The method is validated by comparing the model-predicted results to the experimental data for three real solar cells. The impacts of the series resistance and shunt resistance on the optimum load are also studied and the results show a good consistency with the data reported in the literature. (C) 2008 Elsevier B.V. All rights reserved.