Bifacial PV Rear-Side Gain
The extra output a bifacial module makes from light on its BACK side.
Example
You enter
- Front plane-of-array irradiance (W/m²) 1000
- Rear plane-of-array irradiance (W/m²) 150
- Bifaciality coefficient (0-1, from datasheet) 0.75
- Front-side nameplate power (W) 400
You get
- Bifacial gain 11.25 %
- Effective power 445 W
Details, formula, and sources
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.
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.
The bifacial effective-irradiance relation is public PV engineering practice; the bifaciality coefficient is on the module datasheet and the rear irradiance comes from a site model.
Estimate. AHJ and licensed professional govern.
Field names used by the API: front_poa_wm2, rear_poa_wm2, bifaciality, front_power_w, bifacial_gain_pct, effective_power_w
- Bifacial effective irradiance gain = bifaciality x (rear POA / front POA); effective power = front x (1 + gain)bifacial PV engineering practice
- Bifaciality and albedo phi ~0.65-0.90 from the datasheet; rear POA rises with ground albedo (white membrane ~0.5-0.7 vs asphalt ~0.1), mount height, and row spacingmodule datasheet / site albedo model