Solar Panel Snow Loss Calculator

How to use this calculator

Enter six values and the calculator returns annual kWh lost to snow, percent of yearly production, and a December-to-March breakdown of where the losses fall:

1. System size (kW) — total nameplate of your array.
2. Peak sun hours per day — local annual average from NREL’s PVWatts (3.5 in Seattle, 4.8 in Denver, 5.5 in Phoenix).
3. System efficiency (%) — the derate. PVWatts default is 78%.
4. Panel tilt (°) — fixed angle from horizontal. Roof slope for most installs; latitude-minus-15 for ground mounts in snow country.
5. Annual snowfall (inches) — total winter season snowfall. NOAA’s Climate Data Online gives the multi-year average for any US station.
6. Electricity rate ($/kWh) — your current retail rate, used to value the lost energy.

Why snow loss matters for solar in the US

The US northern tier — from Maine through the upper Midwest, Rocky Mountains, and Pacific Northwest interior — gets 100–300 inches of snow per year. A typical 6 kW Minneapolis array produces about 7,800 kWh annually under PVWatts assumptions, but actual production averages 7,400–7,600 kWh because of snow losses. That gap — roughly 200–400 kWh a year — is worth $30–$70 at average retail electricity rates and is the difference between a 9-year and a 9.5-year payback for most homeowners.

Most online solar quote tools either ignore snow or apply a single 5% blanket derate, which over-penalises tilted ground mounts in Maine and under-penalises low-tilt commercial roofs in Buffalo. Modelling it month-by-month gives a more honest number.

The Marion 2013 NREL snow-loss model

The reference work is Bill Marion’s 2013 NREL paper Measured and Modeled Photovoltaic System Energy Losses from Snow for Colorado and Wisconsin Locations. Marion’s team instrumented test arrays at NREL’s Solar Radiation Research Lab in Boulder and at the University of Wisconsin-Madison, then compared measured production against a clear-sky reference for two full winters.

Three key findings:

1. Sliding is the dominant clearing mechanism. Once the panel surface warms a few degrees above freezing, snow slides off in sheets within minutes. The angle of sliding initiation drops with snow density — fresh dry powder slides at 25–30 degrees, wet spring snow needs 40–45 degrees.
2. Loss compounds with surrounding albedo. When snow on the ground stays for weeks, diffuse irradiance from the bright surface partially compensates for direct losses. This is why high-altitude sites in Colorado sometimes show better February production than December.
3. Bottom-row losses dominate multi-row ground mounts. Snow piles up at the bottom of the array as it slides down panels above. The lowest 4–6 inches of each panel can be covered for 2–3 weeks while the upper portion is clear. Bottom-row losses can exceed top-row by 3–5x on multi-row racking.

US snow loss benchmarks by city

Field data from NREL, NIST, and installer-reported production aligns with these annual snow-loss estimates for a typical 30-degree tilt residential array:

For ground mounts at 40 degrees+ tilt, halve these numbers. For low-tilt commercial roof installs at 10 degrees, double them.

How to reduce snow loss

Increase tilt where possible

Going from 15 to 35 degrees roughly halves snow loss in the Marion data set. For ground mounts in snow country, latitude-tilt (so 45 degrees in Vermont) gives the lowest annual loss when snow and winter sun-angle are both considered. Some racking vendors offer winter-tilt adjusters that let you go to 60 degrees for December-February and return to 25 degrees for summer.

Frameless or rear-bypass panels

Glass-glass frameless modules from LG NeON, REC Alpha, and SunPower Maxeon shed snow noticeably faster than framed panels with aluminium edges. The bottom edge of a framed panel catches a 1–2 inch snow lip that takes hours longer to clear. Worth specifying if you’re in heavy snow country and willing to pay the $0.05–$0.10/W premium.

Bifacial panels in fields

Bifacial panels paired with elevated racking over snow recover 5–10% of annual production from albedo bounce. Ground covered in fresh snow has albedo 0.8–0.9 — nearly perfectly reflective. NREL’s bifacial PV reference yield maps show interior US states gaining the most from bifacial deployment, partly because of winter albedo.

Skip aggressive snow-clearing

Most homeowners over-estimate the value of clearing snow. A 6 kW array producing 200 kWh in January normally loses 30–60 kWh to snow at most. At $0.16/kWh that’s $5–10 a month. Roof falls send roughly 500,000 people to ER each year in the US — the cost of one orthopedic visit dwarfs five winters of recovered solar production.

What the calculator assumes

• Winter (Dec–Mar in northern hemisphere) accounts for 18% of annual production at mid-latitudes
• Snow coverage fraction is calibrated against Marion 2013 Boulder data: 30 cm of monthly snow corresponds to roughly 35% of daylight hours obstructed
• Sliding factor follows 1 − sin(tilt) — drops to 0.43 at 35 degrees, 0.21 at 50 degrees, 0 at 90 degrees
• No correction for rear-of-panel albedo bounce (which would offset 1–2% of loss)
• Assumes monthly snowfall distribution centred on January-February (40% / 30% / 20% of season total)

These assumptions are calibrated for Northern Hemisphere residential arrays in moderate snow climates. Extreme alpine sites or coastal lake-effect snow zones can see actual losses 30–50% higher than the model predicts.

Common mistakes

• Applying a flat 5% snow derate everywhere. A Phoenix install gets 0% snow loss; a Buffalo install at 15-degree tilt gets 8%. A single number obscures siting decisions.
• Forgetting winter is the low-production season anyway. Losing 50% of December’s output (when December is only 4% of annual) is 2% annual loss, not 50%.
• Ignoring snow-guard requirements. Some HOAs and municipal codes require snow guards on solar arrays, particularly over walkways. Allow $0.10–$0.20/W in the budget if you’re in a snow-load county.
• Trusting installer claims of “zero snow loss” at 45 degrees. Marion’s data shows even 60-degree tilts lose 0.5–1% annually in heavy-snow climates. Be sceptical of any number lower than 1%.

Sources

• NREL — Measured and Modeled PV Energy Losses from Snow (Marion 2013) — the reference paper for residential snow-loss modelling
• NREL — System Advisor Model (SAM) — full hourly snow-loss simulation
• NREL PVWatts v6 — annual production baseline before snow adjustment
• NOAA Climate Data Online — multi-year snowfall normals by station
• EnergySage — Solar in Snow Country guide — homeowner-facing snow-loss expectations