Bifacial photovoltaic modules can convert irradiance reaching both the front and rear cell surfaces. Ground mounts are a natural use case because the rear of the module can see reflected light from the ground instead of sitting close to an opaque roof.
The important design point is that bifacial gain is not a universal percentage. Rear irradiance changes with ground reflectance, clearance height, tilt, row pitch, racking shadows, neighboring rows, sun position and diffuse sky conditions. NREL’s bifacial modeling tools treat those variables explicitly, which is a better planning method than assuming a fixed 15%, 20% or 30% uplift.
1. Ground Surface Reflectivity: Understanding Albedo
Albedo is the fraction of incoming solar energy reflected by the ground. It matters, but albedo alone does not tell you the rear-side energy yield. The same surface can produce different rear irradiance when the array height, tilt, pitch or racking geometry changes.
A useful way to think about surfaces is qualitative rather than assigning a guaranteed gain:
| Site condition | Rear-side implication | Design response |
|---|---|---|
| Dark soil, wet vegetation | Usually low reflectance | Do not assume much rear contribution; model the actual layout |
| Grass or ordinary gravel | Moderate, seasonally variable reflectance | Keep the rear optical path open and use a realistic annual albedo input |
| Light aggregate or concrete | Potentially higher reflectance | Compare added surface cost with modeled annual energy benefit |
| Snow-covered ground | Can be highly reflective, but varies with snow age and contamination | Model winter conditions separately and do not assume the front face will remain clear |
NREL’s bifacial tools explicitly model albedo together with tilt, clearance height, row spacing, shading and electrical mismatch. That is the right level of analysis for deciding whether a more reflective ground treatment is worth paying for.
A transparent rear-irradiance example
Suppose a modeled hour produces:
- front plane-of-array irradiance: 800 W/m²
- rear irradiance at the cells: 120 W/m²
- module bifaciality factor: 0.70
A simple first-order equivalent-irradiance calculation is:
In that illustrative hour, the rear contribution adds the equivalent of 84 W/m², or about 10.5% relative to the 800 W/m² front irradiance. That is not an annual-energy claim. Temperature, inverter clipping, spectral effects, mismatch, shading and the full year of sun positions still matter.
2. Mounting Height: Treat Clearance as a Modeling Variable
There is no universal bifacial rule that every array must sit a fixed number of inches above grade. Clearance affects how much ground the rear surface can “see,” but the practical minimum also depends on snow depth, vegetation, access, foundation geometry, wind loading and local structural requirements.
For a design comparison, model at least three cases using the same module and row pitch:
| Case | Lowest module edge | What to evaluate |
|---|---|---|
| Low | Near the minimum mechanically practical height | Rear shading, vegetation maintenance, snow burial risk |
| Medium | Increased clearance | Rear irradiance uniformity and structural cost |
| High | Above expected vegetation/snow constraints | Whether additional rear gain justifies taller structure and wind loading |
Do not optimize bifacial performance in isolation. A taller rack can improve rear exposure while also increasing steel, foundation and wind-load requirements.
Racking matters too. Wide rails, cable trays, torque tubes or dense cross-bracing can cast rear-side shadows. Uniform rear irradiance is preferable because uneven illumination can create cell-level mismatch even when the average rear irradiance looks attractive.
3. Snow: Separate Reflection From Snow-Coverage Loss
Snow creates two different effects that should not be mixed together:
- Snow on the ground can increase reflected irradiance reaching an exposed rear surface.
- Snow on the module can block the front surface and create a separate production loss.
It is unsafe to assume that rear-side electrical generation will warm a module enough to clear the front automatically. Snow shedding depends on tilt, ambient temperature, solar exposure, snow properties, adhesion, frame geometry and whether the lower edge is blocked by accumulated snow. NREL’s snow-loss work treats snow coverage and sliding as their own modeling problem rather than assuming a fixed self-heating benefit.
For winter sizing, use the conservative sequence:
Winter load target
↓
Expected winter solar resource
↓
Front-side snow-loss assumption
↓
Modeled rear irradiance from snow-covered ground
↓
Battery autonomy / backup requirementIf a cabin requires dependable winter power, size storage and backup generation so the design still works during periods when the array is partially or fully snow-covered. Treat any bifacial snow-season gain as modeled upside, not guaranteed rescue energy.
4. A Practical Bifacial Decision Check
Before paying a premium for bifacial modules or ground treatment, compare two modeled systems with the same DC nameplate:
| Question | Monofacial baseline | Bifacial candidate |
|---|---|---|
| Annual modeled kWh | Record baseline | Record modeled increase |
| Rack/foundation cost | Baseline | Include added clearance or structure |
| Ground treatment | Usually none | Include if intentionally changed |
| Snow/vegetation maintenance | Site-specific | Site-specific |
| Rear structural shading | Not applicable | Verify rails and supports |
| Inverter clipping | Check | Re-check with higher modeled DC output |
The useful metric is not “bifacial percentage.” It is additional annual usable energy per additional installed dollar, under the actual geometry and operating constraints of the site.
Design the rest of the generation and storage system with our Solar + Battery Sizing Calculator.