Solar Panel Shading Calculator

How to use this calculator

Enter eight values and the calculator returns annual baseline kWh, annual kWh lost to shading, annual revenue lost, and a recommended mitigation strategy.

1. System size (kW) — total nameplate. A 6 kW system is 6 kW.
2. Peak sun hours per day — local average. NREL PVWatts gives this for any ZIP code; US ranges 3.5 (Seattle) to 6.5 (Phoenix).
3. System efficiency (%) — derate factor. 78% is NREL PVWatts default.
4. Electricity rate ($/kWh) — your retail rate, used to value lost kWh.
5. Total panels in array — count from the design layout.
6. Panels affected by shading — count of panels that get shadow on them during any part of the day.
7. Hours shaded per day — average across the year; pessimistic estimate is fine since winter sun is low anyway.
8. Shading severity (%) — see the FAQ above. A common pattern is 50–70% from trees, 30–40% from chimneys casting diffuse shadow.
9. Inverter topology — microinverter/optimizer, modern string with bypass diodes, or pre-2005 string with no bypass.

How shading really affects solar output

Solar panels are wired in series strings. Current is limited by the lowest-current cell. When light hits a panel, each cell generates a tiny current proportional to the light striking it. If one cell is shaded and produces less current, it acts as a bottleneck — the rest of the string can only push through as much current as the worst cell allows.

Without protection, this is catastrophic: a single bird dropping or fallen leaf could cut output by 90% across an entire 10-panel string. The shaded cell also heats up because it dissipates the power the rest of the string is trying to push through it — a “hot spot” that can burn the back-sheet.

Modern panels solve this with bypass diodes. Every monocrystalline or polycrystalline panel made since roughly 2005 has 3 bypass diodes, one per cell group of 20–24 cells. When light on a cell group falls below ~20% of full irradiance, that cell group’s diode turns on and routes current around it. The string loses 1/3 of one panel’s output instead of the whole string’s output.

Microinverters and DC optimizers take this further: every panel has its own maximum power point tracker (MPPT), so a shaded panel only affects itself. They don’t eliminate shading loss — they just contain it perfectly.

The shading-loss math

For a string with bypass diodes (the common case):

loss_fraction ≈ (shaded_panels / total_panels)
              × (hours_shaded / productive_hours_per_day)
              × (severity / 100)

productive_hours_per_day ≈ 8 (typical PV-productive window around solar noon)

For an older string with no bypass, the entire string drops to the current of the shaded cell during shading hours:

loss_fraction ≈ (hours_shaded / 8) × (severity / 100)

A worked example for a 6 kW system, 4.8 PSH, 78% derate, $0.165/kWh, 2 of 12 panels shaded 3 h/day at 60% severity, modern string inverter:

• Baseline annual kWh = 6 × 1000 × 4.8 × 0.78 × 365 / 1000 = 8,201 kWh
• Loss fraction = (2/12) × (3/8) × 0.60 = 0.0375
• Annual kWh lost = 8,201 × 0.0375 = 308 kWh
• Annual revenue lost = 308 × $0.165 = $50.81

If the same array were on a pre-bypass string inverter, the loss fraction during the 3-hour window would be the full 0.60, and the annual loss climbs to about 22% of the shaded-window output — roughly 7% of annual output.

Typical shading scenarios

Treat soiling as the always-on baseline. Even on a clean array, NREL data suggests 2–5% annual soiling losses in temperate climates, 8–12% in dry/dusty climates like Phoenix or Bakersfield.

Mitigation strategies, ranked by cost-effectiveness

1. Trim or remove trees — usually the highest ROI. A $500–$2,000 tree trim that saves 10% on a 10 kW system at $0.20/kWh pays back in 1–3 years. Don’t trim trees on a neighbor’s property without consent.
2. String layout optimization — wire the unshaded panels in one string and the shaded panels in another. The unshaded string runs at full current. Costs nothing if done at install.
3. DC optimizers (SolarEdge, Tigo) — adds $0.08–$0.15/W. Each panel has its own MPPT but still uses a central inverter. Best for partial shading and mixed-orientation roofs.
4. Microinverters (Enphase IQ8) — adds $0.10–$0.20/W. Each panel is independently inverted. Best for heavily shaded sites, complex roofs, and arrays that will be expanded later.
5. Move the array — if the chosen roof face has 15%+ shading and another face has none, the loss usually justifies the longer wire runs to the other face.

Common mistakes

• Treating all shading as equal. Hard shade (a solid object blocking 90%+ light) is different from soft shade (diffuse light scattered through tree leaves). The calculator’s severity input handles both — be honest.
• Assuming bypass diodes eliminate the problem. They contain it — they don’t eliminate it. A 25% shaded array on a string inverter is still 25% down for the shaded window.
• Ignoring seasonal shading. Winter sun is much lower than summer sun, so a tree that doesn’t shade your roof in June may shade half of it in January. Run the calculator for the worst-case month if shading varies seasonally.
• Overcounting tree growth. Trees don’t grow as fast as people fear — most homeowners overestimate annual height gain. Get a tree assessment from an arborist before deciding to remove vs. trim.

Sources

• NREL — Performance of Photovoltaic Systems with Shading — empirical loss data by topology
• Sandia PV Performance Modeling Collaborative — bypass diode physics and current-voltage modeling
• Google Project Sunroof — free residential shading analysis from aerial imagery
• Aurora Solar — industry-standard shading software used by installers
• SunPower technical brief on cell-level shading tolerance — manufacturer data on bypass-diode behavior