Bifacial PV Rear-Side Gain
The extra output a bifacial module makes from light on its BACK side. The rear cells run at a fraction of front efficiency -- the bifaciality coefficient (phi), a datasheet number ~0.65-0.90 -- and collect the irradiance reflected onto the back. Gain over a front-only module = bifaciality x (rear irradiance / front irradiance): phi 0.75 with 150 W/m^2 rear vs 1000 W/m^2 front = 11.25% more, so a 400 W front rating becomes ~445 W. Rear irradiance climbs with ground albedo (white membrane ~0.5-0.7 vs dark asphalt ~0.1), mounting height, and row spacing -- over a white roof the same module might see 250 W/m^2 (18.75%, ~475 W). A yield estimate; the datasheet bifaciality, the actual site albedo and rear-shading, and a bifacial ray-trace (PVsyst / NREL) govern the real number, and the inverter must be sized for the boost.
Formula and source
bifacial_gain = bifaciality x (rear_poa_wm2 / front_poa_wm2); bifacial_gain_pct = 100 x bifacial_gain; effective_power_w = front_power_w x (1 + bifacial_gain).
Bifacial PV rear-side gain (bifaciality-coefficient effective-irradiance model), by name; the module datasheet bifaciality, the site albedo and rear-shading, and a bifacial ray-trace (PVsyst / NREL) govern the yield.
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