Performance and output

Shading loss estimator

Enter how much of your array gets shaded, how dense that shade is, and when it hits during the day. Get an annual kWh and dollar loss estimate.

Your shading details

1 Project details Optional
Shown on your exported report. Leave blank to skip.
2 Your system
Use your system's rated annual output, or a PVWatts estimate with no shading applied.
Find this on a recent utility bill. US average is around $0.16/kWh.
3 The shading itself
Estimate the portion of panels that get shaded at some point during peak hours.
Denser shade blocks more of the sun's intensity, not just its area.
Shade during midday costs more since that's when sunlight is strongest.
Per-panel electronics reduce how much one shaded panel drags down the rest of its string.

How it works

Three steps to a shading loss number

Step 1

Estimate the shaded share of your array

Walk your roof at midday and note roughly what fraction of panels sit under a shadow at some point. The tree shading calculator helps if a single tree is the main culprit. If a tree is the main culprit, moving the array is often cheaper than new equipment, and the tree line setback sizing tool works out how far back from the tree line the array needs to sit.

Step 2

Rate the shade density and timing

Dense shade during midday costs far more than light dappled shade in early morning. Check the morning vs afternoon tool if you're unsure which window matters more for your roof.

Step 3

Factor in your inverter setup

Microinverters and power optimizers cut the string-wide penalty from shading significantly. If your system uses a standard string inverter, this loss estimate applies at full strength.

Understanding your shading loss

Why shading costs more than the shadow's size

A solar array wired as a standard series string produces current at the rate of its weakest panel. One shaded panel can pull down output from every other panel sharing its string, even ones sitting in full sun. This is why a 20% shaded array often loses more than 20% of its output. The string mismatch loss calculator models this effect on its own if you want a closer look at string-level losses. A NLR field study of shaded PV strings found that shading over 40% of a single cell can zero out the output of its entire bypass diode group, which is why shaded output loss is rarely proportional to shaded area.

Density and timing both change the math

Light, dappled shade from thin branches blocks less of the sun's intensity than a solid shadow from a chimney or a neighboring roofline. Timing matters just as much: shade that only hits before 9 a.m. or after 4 p.m. misses most of the day's actual energy, since peak sun hours concentrate around midday. NOAA's Solar Calculator can trace the sun's exact position and intensity throughout the day for your location, which is a useful way to confirm whether a given shadow actually falls inside or outside your array's strongest production hours.

How microinverters and optimizers change the outcome

A standard string inverter treats a shaded panel as a bottleneck for its whole string. Microinverters and power optimizers give each panel its own conversion point, so a shaded panel underperforms only for itself. Systems built around tight roof space with unavoidable shading often use one of these setups for this reason. See the microinverter vs string inverter output tool for more on choosing between the two.

Seasonal shadows shift throughout the year

The sun's angle changes across seasons, so a tree that clears your roof in summer can cast a shadow across it in winter. If your shading varies a lot by month, the seasonal shading variation calculator models that swing directly instead of averaging it into a single number.

What to do with this number next

Compare your estimated post-shading output against your actual usage with the output vs usage gap analyzer, or check whether trimming or removing the shade source is worth the cost against the kWh you'd recover.

Questions

Frequently asked

A moderately dense shadow covering 20% of your array during peak sun hours typically cuts annual output by 10% to 15%, not just 20%, because shading one panel can drag down its whole string. An 11,000 kWh/year system with 20% moderate shading loses around 1,430 kWh a year.

Panels wired in series strings share current, so one heavily shaded panel can throttle output from every panel in that string, not just itself. This is why a small shadow from a chimney or vent pipe often costs more output than its physical size suggests.

Yes. Shade during midday peak sun hours costs far more output than the same shade in early morning or late afternoon, since midday sun carries the most energy. A tree that only shades your array before 9 a.m. costs much less than one that shades it at noon.

Yes. Microinverters and power optimizers let each panel operate independently, so one shaded panel no longer drags down its whole string. This typically recovers a meaningful share of the loss a standard string inverter would otherwise cause.

Related tools

Keep diagnosing your output

Output loss from tree shading Output loss from partial shading Output loss from chimney shading Morning vs afternoon shading impact Seasonal shading variation String mismatch loss calculator Real world vs nameplate output gap All performance and output tools