We present a new method to characterize bifacial solar cells under standard test conditions (STC). The method considers the bifacial operation of the cell and provides the characteristics for simultaneous front and rear side illumination rather than providing the front and the rear side characteristics separately. The method involves measurements of front side electrical parameters (efficiency, open-circuit voltage, short-circuit current and fill factor) and rear side short-circuit current under STC. Two new parameters are introduced, namely bifacial 1.x efficiency (effective efficiency) and gain-efficiency product, which are calculated from the measured STC parameters. The former provides information related to the cell design considering the bifacial operation, whereas the latter provides the end-use benefits from the modules with bifacial cells for a particular installation. To calculate the bifacial 1.x efficiency and the gain-efficiency product, a one-diode solar cell equivalent circuit is used. Characteristic plots are shown for the newly introduced parameters as a function of rear-side illumination for various example solar cells. A sensitivity analysis is performed to understand the influence of each single-sided STC solar cell parameter on the newly introduced parameters. This sensitivity analysis shows that the fill factor and the rear-to-front current ratio are the most critical parameters for bifacial solar cells. Copyright © 2012 John Wiley & Sons, Ltd.