Bifacial Solar Panel Case Study


This note reports measurements of the output of bifacial photovoltaic panels, both flush-mounted and reverse-tilt-mounted, on a sloped roof in Los Angeles.


Light striking the back side of photovoltaic solar panels is usually wasted. Panels which are "bifacial", or sensitive to light on both sides, are commercially available. Reports of the benefit from back illumination by reflection of bifacial panels tilt-mounted over moderate albedo backgrounds vary between roughly 2% and 10% (see [1-10]). The goals of this note are a) to verify that the front-side output from the bifacial panels is competitive with monofacial panels when flush mounted, and b) to measure the back-side output as a percentage of the front-side output when reverse-tilt-mounted.

Experimental System

A solar system for a single-family house in Los Angeles, originally designed to use 29 monofacial LG310 310 watt panels, was redesigned to use 25 LG310's and 4 bifacial GxB300 300 watt panels.

Panel layout; bifacial panel positions are marked "BF"

The roof is covered in fresh Landmark Solaris Dusky Clay shingles; their datasheet claims an albedo of 0.26.

The panels are connected to a SolarEdge 7600 inverter that also provides per-panel data on panel output and voltage. Readings are taken at random times about once every ten minutes, which makes accurate comparisons difficult. Accuracy is not specified, but is said to be roughly 5%.

This screencast shows the output for each panel during April 23rd, 2016, from dawn to dusk. You can see that the west-facing panels light up later in the day; you can also see the morning and evening shading at work. A thin shadow flits across the array at 8:15am, a tree starts to shade the lower row of the south-facing array at 5pm, and the shadow of a ladder (!) shades panel 1.2.13 after 2pm.

Tilt mounted bifacials

Bifacial panels 1.1.1 and 1.1.2 are reverse tilt mounted, and receive significant indirect light reflecting.

Flush mounted bifacials

The remaining panels are flush mounted 15 cm above the roof. Bifacial panel 1.1.3 is mounted at the corner of the array, and may have significant back illumination. Bifacial panel 1.1.4 is mounted next to it, and has little back illumination.

Mounting Issues


Originally, the installer improvised using a flat roof tilt mount kit, and did not follow the GxB300's data sheet's guidance on placement of mounting clamps, leading to a shaky, saggy mount.
Original mount

On the advice of a mechanical engineer, the installer switched to an Iron Ridge adjustable tilt mount plus a crossbrace, and positioned the clamps properly.

Improved mount


The installer, not realizing these were bifacial panels, mounted the SolarEdge optimizer modules such that they covered much of one or two cells on the back side of each panel.
Optimizers occlude cells
Also, the minimum height above the roof was only about 10cm, and there is very little space between the panels, so very little direct sun reaches the roof beneath the tilted panels at midday.

It is not known how much this affects output. After the current measurements are complete it would be good to correct some of these and re-measure. Possible corrections:


Data will be gathered each Saturday during the test period.

Data will be stored in an online spreadsheet and an online photo album, and overall system performance will be pushed live to

The following data will be gathered:

Temperature readings will be taken with an Extech IR250 IR thermometer pressed directly against the front of the panel; the median of the readings at the center of four cells nearest the desired point will be used. For panels 1.1.1-1.1.2, the desired point is the center of the panel. For panels 1.1.3-1.1.5, the desired point is one cell above the center of the panel (as the center is not reachable).

Each Saturday evening after sunset, one bifacial panel's back side will be covered with Duvetyne (a very dark cloth), and the other panel's back side will be uncovered, such that the cover alternates between the panels each week.

Front view of panels with Duvetyne cover in place on panel 1.1.1
Each time the cover is moved, a timestamped photo will be taken after the move (and possibly before, to document the condition of the cover).


Flush mounted bifacials

Prism Solar defines the term "Bifacial Gain in Energy" (BGE) as follows: BGE = (Etotal / Efront) - 100%, or (Eback / Efront).

BGE for bifacial panel 1.1.3 will be estimated as median(E(1.1.3)/E(1.1.4)) - 100%, where E(x) is daily energy for panel x over the test period. Given that the accuracy of SolarEdge's monitoring is unknown, this won't be terribly accurate.

Swapping the cables to the two panels and repeating might be a way to average out some systematic measurement error, but I haven't checked to see if they'd reach yet.

Tilt mounted bifacials

A key problem in comparing the output from covered vs. uncovered panels is power loss due to heating. Covering the back side of the panel heats it up by about 7 degrees centigrade at midday. The temperature coefficient PMPP for the GxB300 is -0.28%/C, so even with no back light, covered panels should produce 1.7% less energy at midday than uncovered ones.

One approach to compensating for this is to measure only calm, sunny days, and only at local noon, and apply Tc(PMPP) to correct for the temperature difference. This will underestimate BGE, which is largest when the sun is lowest in the sky, by an unknown amount.

To exclude cloudy days, only data for days with a total energy generation of 51kWh or greater and a smooth output curve will be used. To reduce cooling from wind, data will be discarded for days with a wind speed any time during the day of over 10 km/h in the local NWS report.

Raw BGE will be estimated each week as median(P(uncovered)/P(covered)) - 100%, where P(uncovered) is the power measured at 1pm for the uncovered panel, and P(covered) is the power measured at 1pm for the covered panel, corrected for the temperature difference between the two panels.

After an even number of weeks, the raw BGEs for the weeks will be averaged. Averaging alternating readings like this should compensate for both module power differences and measurement equipment miscalibration.

According to Prism Solar's Design Guide for Bifacial Solar Modules v4, for a single row of tilt mounted panels, the expected annual BGE is

 (0.30/deg)*(tilt in degrees) + (11.5/m)*(h in meters) + (0.134/%)*(albedo in percent)
where h is the mininum height in meters (here, 0.1), and tilt is the tilt relative to the surface (here, 40 degrees). The tilt parameter of 40 degrees is out of range; their model only handles up to 35 degrees, so let's substitute that. i.e. BGE = 0.3*35 + 11.5*0.1 + 26 * 0.134 = 10.5 + 1.1 + 3.5 = 15.1%

If observed BGE is underestimated by 50% (due to only being measured when it's at its lowest), and that formula is accurate, we'd expect to observe a BGE of about 7%.




  1. Applications and field tests of bifacial solar modules (2002) said "The yearly-generated power by a vertically installed bifacial module was found to be about 1.4 times larger than that of a vertically installed mono-facial module at the test location."
  2. PV module power gain due to bifacial design. Preliminary experimental and simulation data (2010) said "Non uniformity of back illumination and module elevation are among factors dramatically affecting the energy gain when using bifacial modules."
  3. Performance Investigation of Bifacial PV Modules in the Tropics (2012) found "For the bifacial modules, a performance gain of close to 10% compared to the monofacial modules is easily achievable without modifying the rooftop (reflectivity < 20%)."
  4. Bifacial-PV Power Output Gain in the Field Test using "EarthON" High Bifaciality Solar Cells (2013) measured an 8.6% (over grass) to 23% (over white shells) increase in power due to back side illumination.
  5. Performance of Nine Different Types of PV Modules in the Tropical Region (2014) measured output on a flat, dirty white roof; it didn't directly measure back side contribution, but Sanyo HIT-200DN2-1 (bifacial HIT) outperformed Sunpower SPWR-215 significantly.
  6. Outdoor Performance of Bifacial Modules by Measurements and Modelling (2015) said "For a module with a bifaciality factor of 92% in a location like Amsterdam, this predicted annual yield gain is in the order of 10% at albedo 0.2 and 30% at albedo 0.5."
  7. Relation between Indoor Flash Testing and Outdoor Performance of Bifacial Modules (2014) measured kWh/Wp for east-facing monofacial and bifacial panels at a low-albedo location; bifacial produced 2% more energy in January, and 6% more in July.
  8. Bifacial PV plants: performance model development and optimization of their configuration (2015) said "the gain on the specific production can vary between 7.2 and 14.2% for a bifacial plant when compared with a monofacial plant."
  9. Bifacial solar photovoltaics - A technology review (2016) (paywalled)
  10. Multi-Variable Bifacial Photovoltaic Module Test Results and Best-Fit Annual Bifacial Energy Yield Model (2016)
  11. Design Guide for Bifacial Solar Modules, Prism Solar, 2016

Last Change 24 Apr 2016
(C) Dan Kegel

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