Bifacial Solar Panels and Structural Surveys: Why Glass-Glass Modules Change the Loading Calculation

Bifacial solar panels, panels that generate electricity from light incident on both faces rather than only the front face, have become increasingly common in commercial solar PV installations as panel manufacturers have standardised bifacial cell technology across their product ranges. From a structural engineering perspective, bifacial panels introduce several considerations that are different from monofacial panels and that a structural survey must specifically address.

The structural implications of bifacial panels arise from their physical characteristics, typically slightly heavier than monofacial panels of the same output rating, and from the installation requirements that optimise their bifacial performance: higher tilt angles and elevated mounting to allow ground-reflected light to reach the rear face, which generates larger wind loads on the array.

Weight Implications of Bifacial Panels

Bifacial panels are typically heavier than monofacial panels of equivalent rated output because they require a glass backsheet rather than a polymer backsheet. A standard monofacial 430 Wp panel with a polymer backsheet weighs approximately 20 to 22 kg. A bifacial double-glass 430 Wp panel weighs approximately 25 to 28 kg, an increase of 15 to 35% in panel self-weight.

Converted to dead load per unit area: a monofacial array at 1.7 m x 1.1 m per panel and 22 kg per panel generates a panel dead load of approximately 11.8 kg/m² (0.116 kN/m²). A bifacial glass-glass panel at the same dimensions and 26 kg per panel generates 13.9 kg/m² (0.136 kN/m²). When added to the racking dead load (typically 3 to 5 kg/m²), the total array dead load for a bifacial system is approximately 17 to 20 kg/m², compared to 14 to 17 kg/m² for a monofacial system on the same racking.

For a structural assessment that was conducted against a monofacial specification, switching to bifacial panels post-assessment requires a supplementary structural check to confirm that the increased dead load remains within the structural capacity. Where the original assessment identified the structure as operating with a comfortable capacity margin, the additional dead load of bifacial panels is typically within that margin. Where the original assessment showed utilisation at or near the capacity limit, the additional dead load may require a structural condition change or a reduction in array coverage area.

Tilt Angle Implications for Wind Uplift

Bifacial panel performance optimisation requires higher tilt angles than monofacial installations on flat roofs. A monofacial panel on a flat roof is typically tilted at 10 to 15 degrees to balance performance against wind load. A bifacial panel on the same roof benefits from tilts of 15 to 25 degrees to increase the proportion of ground-reflected irradiance received by the rear face.

Higher tilt angles generate larger wind uplift forces on the array. The BRE Digest 489 wind pressure coefficients for rooftop PV arrays increase with array tilt angle. At 10 degrees tilt, the net uplift coefficient for interior rows of a large flat-roof array is approximately 0.6 to 0.8 (dimensionless). At 20 degrees tilt, the same coefficient is approximately 1.0 to 1.3, a 50 to 60% increase in the uplift coefficient and a corresponding increase in the design uplift force per fixing.

A structural assessment conducted for a monofacial system at 12 degrees tilt cannot be used to confirm fixing adequacy for a bifacial system at 20 degrees tilt on the same roof. The wind uplift calculation is specific to the tilt angle specified in the assessment. If the installation design specifies a different tilt angle from the one assessed, a supplementary wind uplift check is required before the installation proceeds.

Elevated Mounting and Ground Clearance Requirements

Bifacial panels on flat roofs are often mounted at greater heights above the roof surface than monofacial panels, to allow ground-reflected light to illuminate the panel rear face without obstruction from the mounting structure and roof membrane. Elevated mounting typically uses taller ballast frame systems or adjustable height racking rather than low-profile ballasted systems.

Taller mounting systems have structural implications: the increased height increases the wind lever arm for the ballast reaction, increasing the overturning moment on the mounting system and the corresponding ballast weight requirement. A taller system also presents a larger wind face area per panel row, increasing the horizontal wind load on the racking and the anchoring demands on ballast or mechanical fixings.

The structural assessment for a bifacial installation with elevated mounting must account for these geometry-dependent wind load effects. A standard flat-roof ballast calculation developed for a low-profile monofacial system does not account for the elevated mounting geometry of a bifacial system and may significantly underestimate the ballast weight required for wind stability.

Specifying Structural Assessments for Bifacial Installations

When commissioning a structural assessment for a bifacial solar PV installation, the instruction must include the specific bifacial panel weight, the proposed tilt angle, the mounting system type and height, and the proposed array layout including row spacing. These parameters all affect the structural calculation, and a structural assessment produced without them is generic rather than specific to the bifacial installation being assessed.

Where a structural assessment was produced for a monofacial specification and the installation has subsequently been changed to bifacial panels or to a higher tilt angle, the original assessment should be reviewed by the structural engineer to confirm whether the changes fall within the assessed parameters or require supplementary calculation. This review is a standard service that reputable structural engineering firms provide as part of their post-assessment support. It should be obtained before installation commences rather than after the system is in place.

Module-Level Power Electronics and Bifacial Configurations: Combined Loading

High-efficiency commercial installations increasingly combine bifacial panels with module-level power electronics (MLPE), microinverters or DC optimisers attached to individual panels or pairs of panels. This combination affects the structural loading calculation in two ways that a standard single-axis array analysis would not capture.

MLPE devices add weight at panel level: a microinverter typically adds 1.1-1.5 kg per panel, a DC optimiser 0.3-0.5 kg per panel, plus the associated cabling. For a standard 430W panel weighing 22 kg, the addition of a 1.4 kg microinverter increases the panel plus electronics weight by approximately 6%. Across a large array, this incremental weight adds up: a 500-panel installation with microinverters adds approximately 700 kg to the total array dead load compared to the same installation without MLPE, or about 0.01-0.02 kN/m² additional distributed load depending on array density. This may appear marginal, but on buildings where the structural assessment was conducted close to the dead load capacity limit, the MLPE addition could be the factor that converts an unconditional clearance into a conditional one.

The structural implication of MLPE for bifacial arrays is therefore that the structural assessment should be instructed with the MLPE specification included in the total array weight, not excluded as “negligible.” Panel manufacturers publish panel weights that do not include MLPE; the MLPE manufacturer publishes device weight separately; and the installer must confirm the combined panel-plus-MLPE weight to the structural engineer at instruction. This sounds like a minor administrative point, but it is the type of detail that creates MCS audit gaps when the as-installed loading exceeds the specification stated in the structural report.

Racking Elevation for Bifacial Rear Irradiance: Structural Height Implications

Bifacial panels require adequate clearance between the panel rear face and the roof surface to access reflected irradiance from the rooftop substrate. The optimal clearance height for rear irradiance performance depends on the substrate reflectivity (albedo) and the panel tilt angle, but practical values for commercial flat roof installations range from 200mm to 600mm above the roof surface.

Elevated racking configurations have structural implications that standard low-profile racking does not. Higher mounting elevations increase the moment arm for wind loads on the panel, generating greater bending moments at the racking attachment points and higher uplift forces at the upwind racking feet. For the same wind speed and array geometry, a bifacial array mounted at 400mm elevation generates higher wind uplift forces than the same array mounted at 150mm, and the structural assessment must reflect the actual mounting height rather than assuming a standard low-profile installation.

The interaction between elevation height and wind load is captured in the racking manufacturer’s wind load test data, which should specify the uplift coefficient as a function of panel tilt and mounting height. Where manufacturer wind load data is available for the specific configuration, the structural engineer can use it to calibrate the analysis rather than relying on conservative generic coefficients. The developer or installer should provide this manufacturer wind load data to the structural engineer at instruction, particularly for non-standard elevated configurations where generic assumptions would be unduly conservative.

Structural assessment for bifacial arrays should therefore confirm: the total dead load inclusive of panels, MLPE, racking, and fixings at the specified mounting height; the wind load analysis using the elevated configuration geometry; and the clearance verdict against the specific building’s structural capacity at the stated loading. This is a more complete specification than a generic “bifacial panel” assessment that does not account for elevation or MLPE, and it ensures that the clearance documentation reflects the installation that will actually be built.

When instructing a structural assessment for a bifacial PV installation, providing the structural engineer with the confirmed panel weight (inclusive of frame), MLPE device weight per unit if applicable, racking system dead load per unit area, and mounting height above the roof surface at the outset of instruction produces a faster and more accurate assessment than a generic bifacial specification. Panels without MLPE at standard mounting height are assessed differently from high-elevation bifacial panels with microinverters, and the distinction matters for the clearance outcome on buildings with limited residual structural capacity.

WHERE SOLAR SURVEYS ADDS VALUE

Glass-Glass vs. Glass-Backsheet Bifacial: Structural Differences

Bifacial solar panels are manufactured in two primary form factors, each with different structural implications: glass-glass construction (dual-glass) and glass-backsheet construction. Both allow rear-side irradiation but differ in weight, flexibility, and mounting requirements.

Glass-glass bifacial panels use a glass rear sheet instead of the polymer backsheet used in conventional monofacial panels. The second glass layer adds 3-5 kg per panel (equivalent to roughly 1.5-2.5 kg/m²) compared to glass-backsheet equivalents of the same wattage. Glass-glass construction also produces a rigid, brittle panel that is more sensitive to differential settlement or non-planar mounting, the panel cannot flex to accommodate minor irregularities in the mounting surface without risking glass fracture at the corners or edges.

Glass-backsheet bifacial panels have similar weight to monofacial panels of equivalent wattage, because the polymer backsheet is lightweight. The structural assessment implications are essentially the same as for standard monofacial panels of the same physical dimensions and weight.

For structural assessment purposes, the distinction matters because glass-glass panels require more rigorous mounting flatness and levelness tolerances. The mounting structure specification for glass-glass bifacial installations should confirm that the installation tolerance, the maximum permissible deviation from flatness across the panel mounting points, is within the manufacturer's structural warranty conditions. Mounting structures that work fine for glass-backsheet panels may induce unacceptable bending in glass-glass panels.

Rear-Side Access and Structural Survey Implications

The bifacial advantage, rear-side irradiation contributing to overall generation, requires that the rear face of the panels is exposed to diffuse or reflected irradiance. This means bifacial panels are typically installed with a significant gap between the panel underside and the roof surface (minimum 200-400mm in most manufacturer guidance), compared to flush-mounted monofacial panels with minimal clearance.

This increased clearance height has structural implications for wind loading. Panels mounted at greater height above the roof surface are more exposed to under-panel wind pressure, the pressure differential between the windward underside and the leeward upper surface of the panel is greater at higher clearance heights. Wind uplift on bifacial panels installed at 300mm clearance is typically 10-20% greater than the same panels at 100mm clearance, all else being equal.

The structural assessment for a bifacial installation must use the actual mounting clearance height in the wind pressure calculation. Where the bifacial advantage is purchased through increased clearance height, the structural engineer must confirm that the increased wind uplift at that clearance height is within the mounting system's capacity and the roof structural capacity for the relevant zones.

Bifacial in East-West Configurations

Bifacial panels are increasingly specified in east-west configurations on flat commercial roofs, where the east-facing and west-facing panel surfaces both receive direct irradiance at different times of day, and the rear surfaces receive reflected irradiance from the roof membrane between rows. The structural assessment for bifacial E-W systems must address:

• Wind loading at the lower E-W tilt angle (typically 10-15°), which reduces uplift compared to south-facing configurations
• The specific clearance height of the E-W mounting system and its effect on wind pressure coefficients
• Row spacing implications of rear-side irradiance requirements, rows may be more closely spaced than south-facing equivalents, affecting the snow valley drift calculation
• Panel weight per m² of roof coverage, important because E-W configurations cover more of the roof area than south-facing

Future-Proofing: Bifacial Panel Replacement

As bifacial technology matures and becomes increasingly dominant in the commercial solar market, buildings whose original structural assessment was conducted for monofacial panels may subsequently have panels replaced with bifacial equivalents. Asset managers should confirm whether a planned bifacial replacement falls within the original structural clearance by checking three parameters:

• Weight: is the replacement bifacial panel within the weight tolerance specified in the original structural assessment?
• Dimensions: are the panel dimensions within the range specified in the original assessment?
• Mounting clearance: does the bifacial mounting system require greater clearance height than the original monofacial system, and if so, has the wind uplift implication been checked?

Where any of these parameters falls outside the original assessment scope, a structural review is required before the bifacial panels are installed. This is not a significant cost, typically a brief desktop review by the original structural engineer, but it is an important quality control step that prevents the asset from operating with a solar system that exceeds its structurally-assessed load parameters.