In order to accurately calculate the series resistance of solar cells,an analytical expression of the series resistance is derived from the current-voltage characteristic equations of the single diode model of solar cells.The expression is in the form of Lambert W-function.By substituting the current-voltage characteristic curve and other model parameters into the expression,the series resistance can be accurately evaluated,thus avoiding the errors introduced by the commonly-used approximate methods.The accuracy and applicability of the proposed expression is validated by its applications to different solar cells in various working conditions.