*The efficiency of BSCs is usually determined by means of independent efficiency measurements of the front and rear sides under one sun. Sometimes also, the BSC is characterized using its equivalent efficiency defined as the efficiency of a monofacial cell able to render the same power per unit area as the bifacial cell at the same test conditions. Alternatively, the equivalent efficiency has been defined as the sum of the front and rear side efficiencies weighted by the relative amounts of irradiance on both sides.
Another related parameter is the Bifaciality Factor, defined as the ratio of the front and rear efficiencies when illuminated and measured independently.
Also specific to BSCs is the Separation Rate , that intends to measure the Bifacial Illumination Effect predicted by McIntosh et al. in 1997 by which, the electrical output of BSCs operating under bifacial illumination would not necessarily equal the sum of the front-only and rear-only electrical output, i.e. it is not merely a linear combination of the monofacial characteristics:
Typically X represents one of the cell characteristic parameters such as the short circuit current Jsc, the peak power Pmax or the efficiency η. Furthermore, to characterize BSC operation under simultaneous front and rear irradiation, the irradiance gain, g, defined as:
and a bifacial 1.x Efficiency can be defined as the efficiency obtained under a simultaneous irradiance of a certain amount on the front face and x times this amount on the rear side of the BSC. Then the actual gain of a BSC with respect to a monofacial one can be expressed through the Gain-Efficiency Product, which is the product of the irradiance gain g and the bifacial 1.x Efficiency.