In your quote, you may see panels described as “double-sided” or bifacial. Will they produce more electricity than a comparable single-sided panel on your roof? They can, but the additional benefit depends on the amount of light that actually reaches the back side and on the overall system layout.
Before paying more or changing the mounting structure, ask how this benefit was calculated for your property. The technology’s name alone is not enough to predict annual production.
What makes bifacial panels different
Bifacial panels capture light on both the front and back sides of their cells. On a roof, some light is reflected by the roof surface and may reach their back side. They are not two independent panels, and their power output does not automatically double.
In the datasheet, you may see the bifaciality factor. It describes the back side’s relative output compared with the front side under specified conditions. As explained in the PVsyst documentation, this is a panel characteristic. A figure of 80% does not mean your system will produce 80% more kilowatt-hours.
To correctly interpret rated power and reference conditions, also see our guide to photovoltaic panel datasheets.
The back side needs access to light
Consider two different spaces: an open, light-colored roof with suitable clearance beneath the panels, and a roof where the panels are very close to a dark surface. It would not make sense to use the same additional production percentage for both cases.
The bifacial model in SAM software takes into account surface reflectivity, row spacing, and system height. These parameters help describe how much light reaches the back of the panels. Ask for them to be included in your assessment rather than simply adding a general “bonus” to the production estimate.
Also ask how shading from rails, mounting components, and nearby elements is accounted for. The IEA PVPS technical report on bifacial modules and systems examines uneven irradiance on the back surface. The assessment is not limited to shadows visible on the front side.
What reflectivity, or albedo, means
Reflectivity describes the portion of radiation a surface sends back rather than absorbs. For a bifacial assessment, the surface “seen” by the back side of the panels is especially important—not simply the building’s overall color.
The PVsyst notes that surface condition, moisture, and aging affect reflectivity measurements. An initially clean, bright surface should not be treated as an unchanged assumption for all the years to come.
If a proposal includes a new coating, ask for a separate description of the material, cost, and maintenance requirements. Discuss compatibility with the existing roof with the relevant professional. The choice of waterproofing needs its own technical assessment, as we explain in our guide to waterproofing and photovoltaics.
Don’t compare only additional production percentages
First, ask for the basis of comparison to be defined: which single-sided panel, what power rating, which layout, and what shared assumptions are being used? If the number of panels, front-side rated power, and mounting structure all change at the same time, the final difference is not due exclusively to the back side.
A purely hypothetical example: if the baseline estimate is 8.000 kWh per year and a substantiated simulation estimates an additional benefit of 4%, the difference is 320 kWh. These figures illustrate the calculation only; they are not a prediction for a Greek roof or a result for a Solar Home customer.
Ask for the final estimated energy at the system output, after losses, and how it can be used alongside your own consumption. Additional kilowatt-hours do not automatically translate into the same percentage reduction in your bill.
The mounting structure and inverter need to be assessed as a whole
Do not increase the system’s height or spacing on your own so the back side can “see more light.” Ask the engineer to assess the final mounting structure, loads, wind, and available access. A theoretical improvement in one parameter does not prove that a different construction is suitable for your building.
The electrical design also needs to account for the contribution from the back side. The IEA PVPS report specifically examines the higher operating currents in bifacial systems. Ask for confirmation of compatibility with the inverter inputs and the rest of the equipment, without relying solely on the panels’ rated power.
Six questions to ask before approving the quote
- What is the exact panel model number, and which specifications apply to the front and back sides?
- What surface and reflectivity assumption does the assessment use?
- How are heights, spacing, mounting structures, and shading on the back side represented?
- How many additional annual kWh are estimated, and against which reference solution?
- What additional costs arise from panels, mounting, coating or maintenance?
- What result does a more conservative reflectivity scenario produce?
At Solar Home, we assess your site, equipment and needs together. If you’re comparing monofacial and bifacial solutions, you can request a study and quote, so we can discuss which layout makes sense for your installation.
