Solar Shading Loss Calculator
Partial shading rarely costs what it looks like it should. On a traditional string inverter system, shading just 15% of an array for 20% of the day does not cost a proportional 3% of annual output — it can cost closer to 6% or more, because shading even one panel in a series string can throttle the output of the entire string. This calculator estimates that disproportionate annual energy loss from the shaded share of the array, the share of daylight hours affected, and a shading penalty multiplier tied to your inverter/system type. Start with your unshaded production from the Solar Panel Output Calculator, then weigh the shading penalty against the system economics in our Solar Payback Calculator.
The portion of total panel area/count affected by shading from trees, chimneys, or nearby structures.
What share of total daylight hours the shading typically affects the array (e.g., morning shade from an eastern tree line).
Selecting a type auto-fills the Shading Penalty Multiplier below (still editable).
With traditional string inverters, shading even one panel can throttle an entire series string's output, not just the shaded panels -- power optimizers and microinverters largely eliminate this multiplier effect.
% array shaded × (daily shading duration (%) ÷ 100) × shading penalty multiplier
Results update live as you type. For planning and field-check estimates — always verify against applicable standards and equipment ratings.
How we calculate this →Partial shading rarely costs what it looks like it should. On a traditional string inverter system, shading just 15% of an array for 20% of the day doesn't cost a proportional 3% of annual output -- it can cost closer to 6% or more, because shading even one panel in a series string can throttle the output of the entire string, not just the shaded portion. Power optimizers and microinverters largely eliminate this multiplier effect, which is why they're often recommended for roofs with partial shading from trees, chimneys, or neighboring structures.
How solar shading loss is calculated
This calculator estimates the annual energy loss a solar array suffers from partial shading, tying three inputs together: the share of the array affected, the share of daylight hours affected, and a shading penalty multiplier that depends on the inverter/system type. One quantity ties the calculation together.
Estimated Annual Energy Loss (%) = Percentage of Array Shaded (%) × (Average Daily Shading Duration (% of daylight hours) ÷ 100) × Shading Penalty Multiplier. The first term captures how much of the array is shaded — the portion of total panel area or count affected by trees, chimneys, or nearby structures. The second term captures how long that shading persists as a share of total daylight hours (e.g., morning shade from an eastern tree line might affect the array for 20% of the day). The third term, the shading penalty multiplier, accounts for how the inverter/system architecture amplifies or dampens the loss: traditional string inverters process power for an entire series string at once, so shading even one panel can bottleneck the current flowing through the whole string and throttle output well beyond the shaded area alone — a multiplier of roughly 2.0x. Power optimizers sit at each panel and adjust output individually before sending it to a central string inverter, reducing (but not eliminating) the penalty to roughly 1.2x. Microinverters convert power at each individual panel, essentially eliminating the string-wide penalty, so the multiplier falls to 1.0x and losses track the shaded area alone. At the defaults (15% shaded, 20% of daylight hours, String Inverter No Optimizers / 2.0x), that is 15 × (20 ÷ 100) × 2.0 = 15 × 0.2 × 2.0 = 6.0% estimated annual energy loss.
Two notes on the model. First, the multiplier factors are representative planning-level estimates drawn from NREL solar performance modeling and field studies; actual shading losses vary with string length, panel bypass diode behavior, shade geometry, and inverter maximum-power-point tracking. Second, this simplified model does not separately model the time-of-day or seasonal profile of shading — morning or afternoon shade from east/west obstructions affects only part of the day, while south-side obstructions blocking peak midday sun have a much larger annual impact, and deciduous trees shade less in winter than summer. For production estimates used in project financing, validate against a full site-specific shade analysis or NREL's PVWatts tool. Data sources: string inverter shading loss multiplier factors from NREL solar performance modeling and field studies; power optimizer and microinverter performance data from manufacturer specifications and utility-scale/residential solar monitoring; panel series-string current bottleneck effects from IEC 61215 and solar system design standards; seasonal and diurnal shading patterns from solar resource assessment tools and site-specific shade analysis case studies. Verification: with defaults (15% shaded, 20% of daylight hours, String Inverter No Optimizers / 2.0x), Estimated Annual Energy Loss = 6.0%.