TheGreenWatt

Bifacial Solar Panels: How They Work, Gain, And When They're Worth It

Bifacial solar panels generate electricity from both their front and rear surfaces. The front side captures direct and diffuse sunlight like any standard panel, while the rear side converts light reflected off the ground and surrounding surfaces. This dual-sided energy capture typically adds 5-15% more power on ground mounts and 2-5% on rooftop installations.

How bifacial panels work

A standard monofacial solar panel has an opaque backsheet that blocks all light from reaching the rear of the cells. A bifacial panel replaces that opaque backsheet with either a second sheet of tempered glass or a transparent polymer, allowing light to enter from both sides.

The rear cells absorb light that reflects off the ground (albedo light), light that scatters between rows in an array, and diffuse light from the sky that reaches the underside at certain angles. The amount of extra energy depends on three factors: the reflectivity of the surface below, the height of the panel above that surface, and the spacing between rows.

The bifaciality factor on the datasheet tells you how efficient the rear side is compared to the front. A bifaciality factor of 85% means the rear cells produce 85% as much power per unit of irradiance as the front cells. HJT and TOPCon panels achieve 85-95% bifaciality, while PERC bifacial panels reach 65-75%.

Bifacial gain by installation type

Installation TypeTypical Bifacial GainSurface BelowGround Clearance
Flush rooftop mount2-5%Dark shingles (albedo 0.10-0.15)Minimal
Tilted rooftop mount3-7%White membrane (albedo 0.60-0.70)0.3-0.5m
Ground mount, grass5-10%Green grass (albedo 0.20-0.25)0.5-1.5m
Ground mount, white gravel10-15%White crushed stone (albedo 0.40-0.50)1.0-2.0m
Single-axis tracker, desert8-15%Sand/light soil (albedo 0.30-0.40)1.5-2.0m
Snow-covered ground15-25%Fresh snow (albedo 0.80-0.90)1.0m+

Albedo is the dominant variable. Doubling the ground albedo roughly doubles the bifacial gain. For commercial ground-mount projects, spreading white gravel beneath the array is a common and cost-effective way to boost rear-side irradiance by 50-100%.

Glass-glass construction

Most bifacial panels use glass-glass construction: 2.0mm tempered glass on the front and 2.0mm on the rear, versus the typical 3.2mm front glass and polymer backsheet of monofacial panels. This design has several practical consequences.

Lower degradation. Glass-glass modules degrade at 0.3-0.4% per year compared to 0.5% per year for glass-backsheet modules. The glass rear encapsulant blocks moisture and UV better than polymer backsheets, reducing potential-induced degradation (PID) and backsheet cracking over the panel's lifetime.

Longer warranties. Many manufacturers offer 30-year product warranties on glass-glass bifacial panels versus 25 years for standard panels, reflecting the improved durability.

Higher weight. A glass-glass bifacial panel weighs roughly 25-30 kg versus 20-22 kg for a same-size glass-backsheet panel. This adds structural load to the mounting system and may require additional roof reinforcement.

Where bifacial makes economic sense

Bifacial panels dominate utility-scale solar. According to the ITRPV 2024 roadmap, bifacial modules account for over 70% of global utility-scale shipments. The economics work because utility projects use ground mounts with optimized row spacing, controlled ground surfaces, and single-axis trackers that maximize rear irradiance throughout the day.

For residential rooftops, bifacial panels rarely justify their premium. The rear side sits close to the roof surface, receives minimal reflected light, and the 2-5% gain does not offset the higher cost. The exception is residential ground-mount systems or carport installations where the elevation and lighter ground surface can push gains into the 8-12% range.

Related terms

Keep Reading

Frequently Asked Questions

How much extra power do bifacial solar panels produce?
Bifacial gain ranges from 2-5% on a flush rooftop mount to 5-15% on an elevated ground mount with high-albedo surface underneath. In optimal conditions with white gravel or sand beneath the array and wide row spacing, gains up to 20-25% have been measured, but 8-12% is typical for well-designed ground-mount commercial installations.
Are bifacial solar panels worth it for residential rooftops?
For most residential rooftops, bifacial panels offer only 2-5% gain because the rear side is close to the dark roof surface with low albedo. The premium over monofacial panels is often 5-10%, making the payback marginal. They make more sense on flat commercial roofs with white TPO membrane or on ground-mount residential systems.
What is albedo and why does it matter for bifacial panels?
Albedo is the reflectivity of the ground surface, expressed as a decimal from 0 to 1. White concrete has an albedo of 0.30-0.40, green grass is 0.20-0.25, dark asphalt is 0.10-0.15, and fresh snow can reach 0.80-0.90. Higher albedo means more light reaches the rear cells, increasing bifacial gain proportionally.
Do bifacial panels need special mounting?
They need a mounting system that leaves the rear surface exposed. Traditional rooftop flush mounts block rear light almost completely. Tilted mounts with at least 0.5-1.0m ground clearance are ideal. Many utility-scale bifacial installations use single-axis trackers with 1.5-2.0m hub height.
What is glass-glass construction in bifacial panels?
Glass-glass means both the front and rear surfaces are covered with tempered glass instead of a glass front and opaque polymer backsheet. This allows light to enter from both sides and provides better mechanical durability, lower degradation rates (typically 0.3-0.4%/year vs 0.5%/year for glass-backsheet), and longer warranties (often 30 years).
How are bifacial panels rated on a datasheet?
The front-side power is rated at STC like any monofacial panel. Manufacturers also specify a bifaciality factor, typically 65-85% for PERC and 85-95% for HJT and TOPCon, which indicates how efficient the rear side is relative to the front. The actual energy gain depends on installation conditions, not just the bifaciality factor.
Which cell technologies work best for bifacial panels?
HJT and TOPCon have the highest bifaciality factors at 85-95%, meaning their rear side is nearly as efficient as the front. Mono-PERC bifacial panels have a lower bifaciality factor of 65-75% because the aluminum rear contact partially blocks light. N-type cells (TOPCon, HJT) are inherently better suited to bifacial designs.

Sources

Marko Visic
Physicist and solar energy enthusiast. After installing solar panels on my own house, I built TheGreenWatt to share what I learned. All calculators use NREL PVWatts v8 data and peer-reviewed formulas.