Guides · Faisalabad, Lahore, Punjab
Bifacial Gain on White Roofs and Elevated Structures: What You Really Get
Short answer
Bifacial gain is the extra energy a bifacial panel makes from light reaching its back. On a low structure over a dark or dusty roof it is usually small, often only a few percent; on a high structure over a clean, light-coloured surface it can be much larger. Manufacturer headline figures assume near-ideal conditions, so the real gain on your roof depends on three things: how reflective the surface below is, how high the panels sit, and how much of the back is blocked.
For what bifacial and double-glass panels are, read our N-type, P-type and bifacial explainer. This guide is about the practical question: how much extra will you actually get on a Punjab roof?
What bifacial gain means
A bifacial panel produces most of its power from the front. The back adds a top-up from light reflected off the roof or ground, and some diffuse sky light. Bifacial gain is that top-up, expressed as a percentage of what the front alone would give.
Two numbers decide it:
- Bifaciality factor. How well the rear side converts light, compared with the front. It is printed on the datasheet as a percentage. N-type TOPCon and HJT panels are commonly in a higher band than older designs, and some back-contact designs lower. See how to read a datasheet.
- Rear irradiance. How much light actually reaches the back. This is set by the site, not the panel, and it is where most of the difference lies.
Bifacial gain
Front
Direct sunlight
Back
Light reflected from the roof
Height
Elevated structures raise rear gain
The three things that set rear light
1. Albedo: how reflective the surface is
Albedo is the share of sunlight a surface reflects, from 0 (absorbs everything) to 1 (reflects everything). Typical reference values used in design software:
| Surface | Typical albedo | What it means for bifacial gain |
|---|---|---|
| Grass, green fields | about 0.15-0.25 | Low |
| Urban surroundings, mixed roofs | about 0.14-0.22 | Low |
| Grey concrete | about 0.25-0.35 | Moderate |
| Fresh white reflective coating or white tiles | Much higher, product-specific | Highest, while clean |
| Dusty white surface after weeks without cleaning | Falls towards concrete levels | Moderate |
Reference values for screening only. A white coating's real reflectance should come from the product's data or a site measurement.
Many Punjab roofs are dark brick tiles, bitumen or weathered concrete, all at the low end. A white heat-reflective coating raises albedo and also keeps the roof cooler, which some owners want anyway. Coating work must be compatible with your waterproofing; see roof waterproofing with solar.
2. Height: how far the panels sit above the surface
Reflected light needs room to reach the back of the panel. A panel a few centimetres above a roof sees mostly its own shadow. Raise it and the back sees a wider, brighter area of roof, and the shadow spreads out.
| Structure | Typical clearance | Rear-side light |
|---|---|---|
| Low L2 / L3 frame on a flat roof | Low at the front edge | Limited; much of what the back sees is shade |
| Elevated structure (panels above head height) | High | Good, especially over a light surface |
| Carport or canopy | High | Good, but cars and paving below vary |
| Ground mount over gravel or soil | Medium to high | Depends on surface and row spacing |
Our guide to L2, L3 and elevated structures explains each type. Elevated designs also leave the roof usable below, which is often the main reason people choose them.
3. What blocks the back
Rails, cross-members, water tanks, parapet walls and tight row spacing all cast shade on the rear side. A structure designed for bifacial panels keeps rails clear of the cells where possible and spaces rows so the ground between them is not in permanent shadow. A shed roof with the sheet just below the panels gives very little rear light at all.
What the published numbers say
Manufacturers often advertise bifacial gain in the region of 20 to 30 percent, but that figure assumes high mounting, a very reflective surface and open spacing. One panel manufacturer's published field test on a fixed tilt, with modules at least 0.4 m above a white-painted surface, reported a gain of about 20 percent on sunny days. Engineering commentary on typical South Asian commercial rooftops, with low mounting, dark waterproofing and dense rows, puts the realistic gain in the low single digits.
So both claims are true. They describe different installations.
Example arithmetic: low frame vs elevated over a white roof
Example arithmetic only, with round illustrative numbers, not a promise or a measured result. Suppose a 10 kW array would produce 1,300 units a month from its front side in a given season (see units per kW in Punjab to estimate your own).
| Set-up | Assumed bifacial gain | Extra units per month |
|---|---|---|
| Low frame over dark, dusty roof | 3% | about 39 |
| Low frame over clean white roof | 6% | about 78 |
| Elevated structure over dusty concrete | 8% | about 104 |
| Elevated structure over clean white coating | 15% | about 195 |
The point of the table is the spread, not the exact numbers. The same panel can give several times more rear-side energy depending on the structure and the roof.
Is the extra worth paying for?
Bifacial panels are now common in the N-type range, and the price difference from mono-facial versions of the same family is often modest; check current rates on our solar panel price page. The panel itself is rarely the big cost. The cost is in the structure height and the surface treatment.
Ask three questions:
- Do you want an elevated structure anyway? If you want shade or a usable roof below, the bifacial gain is a bonus on top. If you would only raise the panels for bifacial gain, the extra steel and wind design may cost more than the gain is worth.
- Will you keep the surface clean? In dusty Punjab, a white roof loses much of its reflectance within weeks. If the roof will not be washed along with the panels, assume a lower figure. See how often to clean panels in Punjab.
- Will you use the extra units? Under net billing, exported units are credited at a much lower rate than units you buy(/guides/net-metering-net-billing-pakistan/)]. Extra midday output is worth most when it covers your own daytime load. See sizing under net billing.
Structure design matters more for bifacial
Higher panels catch more wind. Double-glass panels are heavier. An elevated structure must be designed for both, with proper foundations or anchoring and bracing for monsoon gusts. Solar Works Pakistan designs structures in-house, and our own teams fabricate and assemble them on site, with a 3-year structure warranty. Read how we approach structure design for Punjab wind and heat. For parking areas and open land, see solar carports and ground-mounted solar.
How to check the gain after installation
If your inverter monitoring shows string-level data, compare a bifacial string over a white area with a similar string over a darker area, on clear days, after both have been cleaned. Differences of a few percent are hard to separate from normal variation, so look at weeks of data, not one afternoon.
Get today's price or a free site survey
Thinking about an elevated structure or white roof coating with bifacial panels? We will assess your roof, height options and wind exposure at the site survey.
- WhatsApp or call: 0336 666 4321
- Email: info@solarworks.pk
- Head office: Main Boulevard, Sargodha Rd, Block F FDA City, Faisalabad
- Lahore branch: 2nd Floor, Jeff Heights, Gulberg, Lahore
- Hours: 9:00 AM to 6:00 PM daily
We serve customers throughout Punjab. Site surveys are normally free. See our installation services or contact us.
Frequently asked questions
How much extra power do bifacial panels give?
It depends on the site. On a low frame over a dark or dusty roof, often only a few percent. On a high structure over a clean, light-coloured surface, it can be much more. Advertised figures of 20 to 30 percent assume near-ideal conditions.
Does painting my roof white increase solar output?
With bifacial panels, it can, because more light reflects onto the back of the panels. The effect is largest with elevated structures and fades as dust settles. With mono-facial panels, a white roof does not add output, though it may keep the roof cooler.
How high should bifacial panels be?
Higher generally means more rear light, because the back sees more of the lit surface and less of its own shadow. The right height balances that gain against structure cost, wind loading and how you want to use the roof. We decide it at the site survey.
What is the bifaciality factor?
It is how well the back of the panel converts light compared with the front, given as a percentage on the datasheet. A higher factor means more gain from the same rear light. Check it for the exact model quoted.
Are bifacial panels worth it on a factory shed roof?
Usually the rear gain is small, because the metal sheet sits just below the panels and blocks most reflected light. Choose bifacial there only if the price is the same as mono-facial or you want double-glass durability.
Does dust reduce bifacial gain?
Yes. Dust on the roof lowers its reflectance, and dust on the rear glass blocks light. In Punjab, keeping both the panels and the surface below them clean makes a noticeable difference.
Sources
- Panel manufacturer (Trina Solar) technical article, "Albedo: a key factor for assessing bifacial gain": https://dpo.trinasolar.com/eu-en/Albedo-key-factor-bifacial-gain
- SurgePV, "Albedo values for solar design"
- Secured Engineers, "Bifacial solar panels: do they pay back in India?": https://www.securedengineers.com/insights/bifacial-solar-panels-do-they-pay-back-india/