Albedo
Albedo is the fraction of sunlight a surface reflects.
| Surface | Typical albedo |
|---|---|
| Fresh snow | 0.80–0.90 |
| Old or wet snow | 0.45–0.70 |
| Light sand, white gravel | 0.30–0.45 |
| Concrete | 0.25–0.35 |
| Grass, fields | 0.15–0.25 |
| Asphalt, dark soil | 0.05–0.12 |
How much a tilted panel receives
Gground = GHI × ρ × (1 − cos β) / 2
GHI is the light falling on horizontal ground, ρ the albedo and β the tilt. The factor (1 − cos β)/2 is the share of the panel's view that is ground: 0 for a flat panel, 9% at 35°, 33% at 70° and 50% for a vertical wall.
Example: 100 W/m² of winter light on the ground and fresh snow at 0.8. A 35° panel gets about 7 W/m² extra; a 70° panel about 26 W/m²; a wall 40 W/m². On a bright winter day the steep panel's ground term alone can match what a flat panel collects from the whole sky.
In our model
Our calculators use an albedo of 0.2 by default and switch to 0.4 in months with a mean temperature near freezing and 0.65 in months below −2 °C — a conservative stand-in for snow cover. The switch can be turned off in the off-grid calculator.
Snow on the panel
The reverse effect matters as much: snow lying on a panel blocks nearly all light. Steep panels shed snow quickly; panels at 30° or less can stay covered for days after a snowfall. The steep tilt helps twice — it sees more reflected light and it is clear of snow sooner.
Bifacial panels
Bifacial modules collect light on the back as well. Mounted vertically in an east–west or north–south row over snow, they can outperform monofacial panels in winter by 20% or more. The same albedo arithmetic applies to both faces.