Bifacial Modules on a Factory Roof: When the Rear-Side Gain Is Real

Bifacial modules gain extra energy only when light reaches their rear face, and on most factory roofs it reaches it weakly. A light-coloured, flat roof with modules raised well above it can give a worthwhile gain. A dark roof, or a metal-sheet roof with modules close to the sheet, often gives little. Test the gain with a site-specific simulation before you pay the premium.

What decides the rear-side gain?

A bifacial module has a front side and a rear side that also converts light. The datasheet gives a bifaciality coefficient, the ratio of rear-side to front-side output. The BIS-hosted test report format for IS/IEC 61730-2 defines it as the ratio of the main I-V characteristics of the rear side to those of the front side, typically at standard test conditions. That coefficient is a property of the module. The gain you actually get depends on how much light the rear face receives, which is a property of your roof and your layout:

• Albedo, the reflectance of the surface beneath and behind the module.
• Height of the module above that surface.
• Row spacing and tilt, which control how much sky and reflected light can reach the underside.
• Obstruction by rails, purlins, ballast blocks and cable trays.
• Cleanliness of the surface and the rear glass.

The same BIS test report format says that for bifacial modules the safety factor on conductor current may be adjusted on the basis of a full simulation of the location and orientation, and for bifacial modules that includes ground albedo, row spacing and installation height. In other words, even the standards expect the rear contribution to be modelled, not assumed. See the BIS report.

Which factory roofs suit bifacial modules?

The "gain" column is qualitative on purpose. Published bifacial gains vary widely with all the factors above, and we could not find an Indian regulator or ministry source that states a rooftop figure. If a proposal quotes one, ask for the albedo, height and row spacing behind it.

How do height, tilt and layout change the answer?

Capacity and rear gain pull in opposite directions on a fixed roof. Wider row spacing and a steeper tilt let more light reach the underside, but they hold fewer modules per square metre of roof. A tightly packed low-tilt array can host more kilowatts yet leave the rear face almost in shade. A mono-facial array packed densely may out-produce a bifacial array spaced widely on the same roof, because the roof is the limit. Compare the two layouts on annual kWh for the whole roof, not on watts per module.

The electrical design also changes. The module's short-circuit current is reported at more than one irradiance level, so strings, cables and protection are sized against the higher value, and the inverter may clip on bright days. Ask your designer to show the DC/AC ratio and the expected clipping loss with and without the rear contribution. Our inverter architecture note covers the sizing side.

Is the cost premium worth paying?

The test is simple. The extra annual energy must exceed the extra plant cost. A first-order check: the plant premium is the module price premium times the module's share of total plant cost. Illustration only: if bifacial modules cost 5% more and modules were 40% of the plant cost, the plant would cost about 2% more, so the rear side must add at least about 2% to annual energy before financing and any extra cabling just to break even. Replace those inputs with your quotation's. If the simulated gain barely clears the break-even, choose the mono-facial module and keep the simplicity.

Two practical points. Bifacial products are often glass-glass and usually heavier than glass-backsheet, so the roof load and the clamp design need to be checked; our note on solar on a PEB sheet roof covers purlin loads. And the model you pick has to be on the Approved List of Models and Manufacturers where it applies: MNRE's O.M. of 4 August 2026 keeps ALMM List-I in force for net-metering and open-access projects (O.M.).

When does mono-facial win, and when does bifacial?

• Mono-facial wins on metal-sheet roofs with modules close to the sheet, on dark roofs, on heavily shaded or tightly packed layouts, on roofs with weight limits, and where you cannot keep the roof clean.
• Bifacial wins on reflective flat roofs with raised frames, on carports and elevated shed-top structures, and on ground-mount over a light surface, where the simulation shows a gain above break-even.
• Either can win where the numbers are close. Then warranty terms, supply and the maker's standing decide. See TOPCon and N-type modules and our earlier bifacial payback explainer.

What should you ask a solar contractor to show?

1. The simulation inputs: roof albedo, module height, tilt, row pitch, bifaciality coefficient.
2. Annual kWh for the bifacial and the mono-facial option on the same roof layout.
3. Whether the yield figure in the proposal includes the rear gain, and whether any generation guarantee depends on it.
4. The module weight, the clamp or ballast design, and the structural certification for the roof.
5. The cleaning plan for the roof surface and the access route.

How Secured Engineers helps

We design the plant in-house and run both module options through the same layout so you can compare kWh, not brochures. Our Chennai project record, a 120 kW plant on a metal-sheet industrial roof with rail-mounted modules near the roof plane, shows the kind of low-clearance roof where this question arises; the record does not state the module type, and we do not claim a bifacial result from it (see the project page). Fire NOC, CEIG and DISCOM net-metering filings are done by our own team, and solar can sit in one contract with your MEPF scope. We work from offices in Noida (Delhi NCR) and Ludhiana, with 535+ projects across 18+ states and 89.77% repeat clients.

Send us the roof type and a photograph and we will say whether bifacial is worth simulating. Read about solar PV modules, message us on WhatsApp, call +91 70099 85817, or ask for a free proposal.