Bifacial Gain Explained: What 5, 10 and 20 Percent Rear Irradiance Actually Deliver
Bifacial Gain Explained: What 5, 10 and 20 Percent Rear Irradiance Actually Deliver
Bifacial marketing often says \”up to 25 percent more power\”. The number is real, but it is not a property of the module alone. It depends on how much light reaches the back, the bifaciality of the cells, the mounting height and the surface below. Two datasheets can use the same percentage label and mean different things. This article shows the actual tables from two current modules and explains how to read them without overstating the gain.
Three numbers that are often confused
Bifaciality factor
Bifaciality compares the rear-side efficiency with the front-side efficiency. A module with 80 percent bifaciality produces about 80 percent as much power from the same irradiance on the back as it would from the front. The Trina 665 W datasheet lists 80 percent plus or minus 10 percentage points; the Jinko 605 W datasheet lists 80 percent plus or minus 5 percentage points. The tolerance matters when you compare modules.
Rear irradiance ratio
This is how much light reaches the back of the module compared with the front, usually written as 5, 10 or 20 percent. It depends on albedo, mounting height, tilt, row spacing and shading. It is a site property, not a module property.
Power gain
Power gain is the increase in total output compared with the front-only rating. It is the number that matters for energy yield, and it is not the same as the rear irradiance ratio. A 20 percent rear irradiance ratio does not automatically produce a 20 percent power gain.
Case one: Canadian Solar 665 W bifacial table
The Canadian Solar CS7N-665MB-AG datasheet lists the front rating as 665 W, then gives bifacial outputs at three rear-side conditions:
| Rear-side condition in the datasheet | Module output | Increase over 665 W | Actual power gain |
|---|---|---|---|
| 5 percent | 672 W | 7 W | approximately 1.05 percent |
| 10 percent | 704 W | 39 W | approximately 5.9 percent |
| 20 percent | 768 W | 103 W | approximately 15.5 percent |
The labels here describe rear-side irradiance, not total power gain. Even at the 20 percent condition, the module produces approximately 15.5 percent more than its front rating, not 20 percent. This is the most common misreading of bifacial datasheets.
Case two: LONGi 645 W bifacial table
The LONGi Hi-MO9 LR7-72HYD datasheet takes a 645 W front reference and lists output at five rear-side gain levels. In this table the labels are actual power gain percentages:
| Label in the datasheet | Module output | Increase over 645 W | Actual power gain |
|---|---|---|---|
| 5 percent | 677 W | 32 W | approximately 5.0 percent |
| 10 percent | 710 W | 65 W | approximately 10.1 percent |
| 15 percent | 744 W | 99 W | approximately 15.3 percent |
| 20 percent | 776 W | 131 W | approximately 20.3 percent |
| 25 percent | 808 W | 163 W | approximately 25.3 percent |
Here the labels match the actual gain closely, because the manufacturer has already converted the rear-side conditions into a power gain for this specific module and mounting assumption. That is why you cannot compare a \”20 percent\” column from one datasheet with a \”20 percent\” column from another without reading the heading.
Why the two tables behave differently
The gain depends on the bifaciality of the cells, the rear-side irradiance, the height of the module above the surface, the tilt, the row spacing and the reflectivity of the ground or roof. A table is calculated for a defined set of those conditions, and the heading should state them. When the heading is missing from a sales presentation, ask for the full datasheet page.
A rough first check is to multiply the bifaciality by the rear irradiance ratio. A module with 80 percent bifaciality under 10 percent rear irradiance would gain roughly 8 percent, before losses. This is only a sanity check. For a bankable number, use the manufacturer bifacial table or a bankable energy model that includes the site albedo and mounting geometry.
When bifacial gain is worth paying for
- Ground-mount on light gravel or sand. High albedo and elevated mounting give the rear side something to work with.
- Elevated structures and carports. The gap under the module allows reflected light to reach the back.
- White or reflective roof membranes. Useful, but usually less than ground-mount because the mounting height is lower.
When it is not
- Flat-mounted modules on dark membrane roofs. Very little light reaches the back, so the gain can be close to zero.
- Roofs with heavy shading between rows. Shading affects the rear side as well as the front.
- Projects where the extra weight and glass cost outweigh the energy gain. Dual-glass modules are heavier, and the mounting structure must be designed for that weight.
How to use this in a tender
- Ask each supplier for the full bifacial table with the heading that defines rear irradiance, height and albedo.
- Do not compare a rear-irradiance column with a power-gain column. Normalise both to actual power gain at the same rear condition.
- Ask for the bifaciality tolerance. A module rated at 80 percent plus or minus 10 points is a wider range than one rated at 80 percent plus or minus 5 points.
- Model the gain for the actual site, not for the best-case marketing condition.
Industry watch: two verified items that affect module choice
Cell technology and end-of-life rules keep moving. Both affect how much weight you should give to a single bifacial number in a tender.
- Cell technology is still shifting. pv magazine reported on 30 October 2026 that SEG Solar plans a 4 GW HJT cell manufacturing plant in Texas. HJT, TOPCon and back-contact cells do not share the same bifacial behaviour, so a bifacial table from one cell platform cannot be transferred to another. Source: pv magazine, 30 October 2026, https://www.pv-magazine.com/2026/10/30/seg-solar-plans-4-gw-hjt-cell-manufacturing-plant-in-texas/
- Recycling costs are becoming part of the business case. pv magazine reported on 30 October 2026 that a new framework identifies the key costs in PV panel recycling. Dual-glass modules have a different end-of-life handling profile from single-glass modules, which matters for projects that model lifetime cost rather than only module price. Source: pv magazine, 30 October 2026, https://www.pv-magazine.com/2026/10/30/new-framework-identifies-key-costs-in-pv-panel-recycling/
Frequently asked questions
Does a 20 percent bifacial gain mean 20 percent more power?
Only if the datasheet heading says the label is a power gain. In the Canadian Solar 665 W table, the 20 percent label refers to rear-side irradiance and the actual gain is approximately 15.5 percent. In the LONGi 645 W table, the 20 percent label is a power gain and the actual increase is approximately 20.3 percent.
What is a realistic gain for a commercial rooftop?
It depends on mounting height and the roof surface. Flat-mounted modules on a dark membrane often see very little gain, while elevated mounting on a light surface can capture more. Use the manufacturer table for the actual mounting geometry rather than a generic percentage.
Is a bifacial module always better than a monofacial module?
No. Bifacial modules add weight and usually cost more. If the site cannot deliver meaningful rear-side irradiance, a monofacial module can be the better economic choice even though its headline efficiency is lower.
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