Bifacial solar panels collect light on both sides, and in early field results the National Renewable Energy Laboratory reported annual energy gains of about 6.5% for bifacial PERC modules and 9% for bifacial heterojunction modules compared with their front side alone. Those results, from NREL’s 75kW bifacial tracker field in Golden, Colorado, show what the rear side adds in a real installation, while makers of portable panels quote higher best-case numbers: EcoFlow says its NextGen 220W bifacial panel can “collect up to 25% more energy.” The gap comes down to one thing, how much light reaches the back. This guide explains how bifacial gain works, what the specs mean and when it pays off.
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Sources: field results are from NREL’s December 2019 presentation “Understanding Bifacial Photovoltaics Potential: Field Performance” (Deline et al.). Panel specs are from EcoFlow’s NextGen 220W Bifacial page and the 220W Bifacial user manual. Gains I calculate myself are labelled as estimates.
Bifacial vs monofacial in one table
| Monofacial | Bifacial | |
|---|---|---|
| Active sides | Front only; opaque backsheet | Front and rear; clear back (glass or transparent sheet) |
| Extra output | None | Depends on light reaching the rear |
| Best mounting | Flush on a roof or against a surface | Raised, with open space and bright ground behind |
| Typical use | Rooftops, RV roofs, flat mounts | Ground mounts, carports, portable kickstands |
| Rated power label | One number | Front rating, often plus a rear rating or bifaciality |
How bifacial gain works
The front of a bifacial panel works like any other. The rear picks up light reflected from the ground and scattered from the sky. How much power the rear adds depends on three things:
- Bifaciality: how efficient the rear is compared with the front. NREL defines it as rear power divided by front power under the same light.
- Ground reflectivity (albedo): bright surfaces such as snow, white gravel or a light tarp reflect much more than dark soil or asphalt.
- Height and shading: the higher a panel sits and the less its own frame, stand or neighbors shade the ground behind it, the more reflected light reaches the back.
NREL’s field slides show how much shading matters: in a long row of modules, the interior panels receive noticeably less rear irradiance than a single panel or the end of a row, a self-shading effect of up to about 50% in their chart.
What NREL measured
| Technology (NREL single-axis tracker field) | Annual bifacial energy gain |
|---|---|
| PERC bifacial modules | About 6.5% |
| Silicon heterojunction (Si-HJT) bifacial modules | About 9% |
NREL described these as initial results from about six months of operation, and noted that its models appeared conservative compared with the rear-side light it actually measured. Its monthly charts also flag snow periods, when bright ground boosts rear-side output. Two takeaways: real-world gains over ordinary ground are modest but steady, and they climb when the surface behind the panel is bright.
NREL’s conclusion slide adds that energy gain “depends on the site configuration and surface albedo,” and that modeling tools such as SAM and PVsyst can estimate it for a specific site.
Reading a bifacial panel’s specs
EcoFlow’s NextGen 220W Bifacial is a good example because it lists both sides:
| Spec | EcoFlow NextGen 220W Bifacial | EcoFlow 220W Bifacial (original, per manual) |
|---|---|---|
| Rated power | 220W front / 175W rear | 220W front / 155W rear |
| Cell type | N-type TOPCon | P-type PERC |
| Efficiency | 25% | 22–23% |
| Bifaciality | About 80% (175/220, my calculation) | 70% ±10% (listed) |
| Open-circuit voltage | 21.5V (Vmp 18.4V) | 21.8V (Vmp 18.4V) |
| Short-circuit current | 12.4A front / 9.9A rear | 13A front / 8.8A rear |
| Weight | About 15.4 lb (7.0kg) | About 20.9 lb (9.5kg) |
| Stand | Integrated 30–60° stand with angle guide | Case doubles as a 30–80° kickstand |
| Water rating | IP68 | — |
| Price (EcoFlow, Oct 17, 2026) | $399 | — |
The rear rating is what the back would produce if it faced full sun. EcoFlow’s manual says just that: you can generate power with the rear facing the sun, but only about 80% as much as the front. In normal use the rear sees only reflected light, so it adds a fraction of its rating, not the full 175W.
Estimating your gain
A simple rule of thumb: extra output is roughly bifaciality times the share of light reaching the back, compared with the front. Using the NextGen panel’s roughly 80% bifaciality:
| Rear light vs front (assumed) | Example surface | Extra output (estimate) | On a 220W panel (estimate) |
|---|---|---|---|
| 5% | Dark soil, panel close to the ground | About 4% | About +9W |
| 10% | Grass or dirt, panel on a kickstand | About 8% | About +18W |
| 20% | Light sand, concrete or a white tarp | About 16% | About +35W |
| 30% | Fresh snow, panel raised | About 24% | About +53W |
These rows are illustrations of the math, not measurements. They show why EcoFlow’s “up to 25%” is a best case for bright, open surroundings, and why NREL’s field results over typical ground land in the single digits.
When bifacial is worth it
- Portable panels on a kickstand: the back is open to the ground, so you get some gain almost anywhere. Set the panel over light sand, a concrete pad or a light tarp for more.
- Ground mounts and carports: raised panels with open space behind them suit bifacial well.
- Snowy climates: NREL’s charts highlight snow months as a period of higher gain.
- Boats and beaches: bright surroundings can help, and EcoFlow’s NextGen panel carries an IP68 rating.
When monofacial makes more sense
- Flush roof mounts: on an RV roof or a house roof, the back faces the roof, so the rear adds little. See our guide to installing flexible panels on an RV roof.
- Flexible panels glued down: the back is covered entirely. Our flexible vs rigid comparison covers that choice.
- Tight budgets per watt: if the panel will sit flush anyway, spend on more front-side watts instead.
Getting the most from a bifacial panel
- Aim the front at the sun. The front still does most of the work; tilt matters more than the rear. Our tilt and angle guide explains how to set it.
- Raise it. More height lets more reflected light reach the back.
- Brighten the ground. A light-colored tarp or ground sheet behind and under the panel is an easy test.
- Keep the back clear. Don’t lean bags, bins or the stand’s case against the rear.
- Leave gaps between panels. Side-by-side panels shade each other’s ground; NREL’s data on interior rows shows the effect.
Bifacial vs a large monofacial portable panel
Another way to compare is by what you carry. EcoFlow’s 400W portable panel is monofacial: its product listing gives 23% efficiency, a 35.3 lb weight, an IP68 rating and a case that folds out into a stand adjustable from 40 to 80 degrees.
| Spec | NextGen 220W Bifacial | EcoFlow 400W (monofacial) |
|---|---|---|
| Front rating | 220W | 400W |
| Weight | About 15.4 lb | 35.3 lb |
| Watts per pound, front only (my calculation) | About 14W/lb | About 11W/lb |
| Rear-side bonus | Yes | No |
Two NextGen 220W panels give you 440W of front rating plus a rear-side bonus for about 31 lb, against one 400W monofacial panel at 35.3 lb. The pair also splits into two lighter loads to carry and can be aimed separately. The single 400W panel is simpler to set up and uses one cable. Either way, the front rating is still what sizes your system; treat the rear as a bonus.
Wiring and power station limits
A bifacial panel’s rear side adds current, not voltage. On the NextGen 220W, EcoFlow lists 12.4A short-circuit current for the front and 9.9A for the rear, so check your power station’s input current limit when you wire panels in parallel. Our solar panel specs explainer covers Voc, Isc and temperature effects, and series vs parallel wiring covers how to combine panels.
FAQ
How much more power do bifacial panels produce?
It depends on the surface behind them. NREL measured about 6.5% (PERC) and 9% (heterojunction) annual gains on trackers over typical ground. Makers quote best cases up to about 25% over bright surfaces.
Are bifacial panels worth it on an RV roof?
Usually not, because the back faces the roof. They shine on kickstands, ground mounts and carports.
What does “bifaciality” mean?
Rear-side power divided by front-side power under the same light. EcoFlow lists 70% ±10% for its original 220W bifacial panel; its NextGen panel’s 175W rear rating works out to about 80%.
Can I flip a bifacial panel to face the rear toward the sun?
You can, but EcoFlow’s manual says the rear produces only about 80% as much power as the front.
Does a white tarp help?
It should, because brighter surfaces reflect more light onto the back. How much depends on height and shading, so it’s worth testing with your own panel.
Bottom line
Bifacial panels add free energy from the rear side, but the amount depends on what’s behind them. Over ordinary ground, expect single-digit gains like NREL’s 6.5–9%; over snow, sand or a white tarp with the panel raised, the gain can climb toward makers’ best-case figures. For a portable panel on a kickstand, bifacial is an easy win; for a flush roof mount, put your money into front-side watts. You can find EcoFlow’s 220W bifacial panel on Amazon.