Bifacial PV modules

Rear-side generation, rating and site-specific yield.

A bifacial PV module can convert light reaching both the front and rear of its cells. The front receives direct and diffuse sky light. The rear mainly receives light reflected from the roof, ground and surroundings, plus diffuse light that reaches it around the module.

The extra energy is not a fixed percentage attached to the module. It is a system yield produced by the module, mounting geometry and site together.

Bifaciality is not rear-side gain

The bifaciality factor compares rear-side and front-side electrical response under controlled test conditions. It describes the module. Rear-side gain compares energy from an installed bifacial system with the energy expected without the rear contribution. It describes an installation and varies with time.

A high bifaciality factor cannot compensate for a rear surface that sees almost no light. Conversely, an open elevated array over a reflective surface may obtain useful rear contribution from a module with a lower factor. Proposal software should model the geometry and state its rear irradiance assumptions.

What controls rear irradiance

Important variables include:

surface reflectance, including how it changes when wet, dirty or aged
clearance between module and surface
tilt, orientation and row spacing
shading from adjacent rows, frames, rails, cables and junction boxes
array edge effects and the position of each module within the field
surrounding buildings, parapets and vegetation
mismatch between modules receiving different rear irradiance

On a flush or in-roof domestic array, the rear face is close to an opaque roof and rear contribution is usually negligible. A bifacial module can still be selected for its price or construction, but the yield forecast should not add a generic bifacial bonus.

Flat roofs need particular care. A light-coloured membrane may be reflective when new, but ballast, dirt, standing water, maintenance routes and close row spacing can change what the rear sees. Raising or spreading rows can improve rear light while increasing wind loading, roof use and mounting requirements. The complete roof design decides the sensible trade-off.

Rating and inverter design

The ordinary nameplate power is based on defined front-side test conditions. Bifacial standards provide methods for characterising rear response and equivalent irradiance. Datasheets may show front rating, bifaciality coefficients and additional outputs at assumed rear irradiance.

Those additional-output rows are scenarios, not a promise for the roof. The design should state which value is used for annual yield and which value is used for electrical maximums.

Rear irradiance can increase module current. Cable, connector, optimiser and inverter current limits, and any string overcurrent protection, must therefore use the applicable bifacial design values rather than front-side power alone. Voltage behaviour and cold-string checks still apply in the normal way.

Construction and mounting

Many bifacial products are glass-glass, but the terms are not interchangeable. Bifacial describes active rear cells. Glass-glass describes encapsulation. The module may instead use a transparent backsheet.

Mounting components should avoid unnecessary rear shading and stay within approved clamp zones. Changing to frameless or glass-glass construction can change clamp, handling, earthing, loading and fire evidence. The exact module and mounting-system instructions govern.

How to evaluate a quoted gain

Ask for:

module bifaciality and rating method from the datasheet
the assumed rear irradiance or bifacial gain in the yield model
surface reflectance and the source of that assumption
array clearance, tilt, row spacing and rear obstructions
treatment of dirt, ageing, mismatch and edge effects
current checks at the maximum modelled rear contribution
a yield comparison using the same front-side module assumptions with the rear contribution removed

If the installer cannot show these inputs, the additional yield should be treated as an unproven upside rather than included in the financial case.

Applies to

Solar

Last reviewed

22 Jul 2026