System sizing

Partial roof shading sizing adjustment

A chimney, tree, or neighboring roofline doesn't just cost you the shaded area. See the real production loss for your inverter type, and the extra panels that close the gap.

Your numbers

1 Project details Optional
Shown on your exported report. Leave blank to skip.
2 Your shading pattern
Estimate the portion of your array's total area that gets shaded at some point during peak sun hours.
The share of sunlight blocked on the shaded panels. Solid shade from close structures blocks more than soft, distant shade.
3 Inverter type
String inverters wire panels in series, so shade losses spread across the whole string.
The system size you'd hit without any shading loss.
4 Panel choice
Most home panels quoted in late 2025 were 400 to 460 watts, according to EnergySage's marketplace data.

How it works

Three steps to your adjustment

Step 1

Estimate your shaded area and severity

Walk the roof at different times of day, or note where a chimney, tree, or neighboring structure casts a shadow across your planned array. Soft, distant shade behaves differently than solid, close-range blockage.

Step 2

Factor in your inverter architecture

String inverters spread shading losses across the whole string, while microinverters and power optimizers isolate the loss to just the shaded panel. In NREL's testing, a string setup produced about 4% to 12% less annual energy than microinverters under light to heavy residential shade. See the DC to AC ratio calculator if you're still finalizing inverter sizing.

Step 3

Add panels or trim the shade

Adding panels restores your target output on paper, but trimming a tree or moving an obstruction fixes the underlying cause. Compare against the multi-roof plane array splitter if splitting the array across a shade-free section helps more.

Why shading costs more than the shade itself

The series-wiring problem with string inverters

A standard string inverter connects panels in series, so the string's current is limited by its weakest panel. That is why shade costs more than the light it blocks. NLR's test of a partially shaded grid-tied string found that in one configuration a shadow cut power by more than 30 times its own size. That is a worst case. Bypass diodes route current around shaded sections, and shade moves through the day, so typical annual losses are smaller, but a string still loses more than the sunlight that was blocked.

What microinverters and optimizers actually recover

NREL's shading testbed report measured the gap directly, using shade patterns drawn from more than 60 residential installations. On identical 8 kW arrays, the microinverter system produced 3.7% more energy under light shading (a 7% irradiance reduction), 7.8% more under moderate shading (15 to 19%), and 12.3% more under heavy shading (25% or more) than the string inverter. Neither technology can generate power from blocked sunlight, so some loss always remains. This calculator uses those measurements: at the example values, a 15% shaded area at moderate severity blocks about 9.0% of annual light, which costs a microinverter system 9.0% and a string system about 12.9%.

When the microinverter premium is worth paying

At NREL's measured gains, a microinverter or optimizer system earns about 4% more under light shade and about 8% more under moderate shade. Whether that justifies the extra equipment cost depends on your quotes, a comparison the power optimizer vs microinverter comparison breaks down further. With very little shade there is little loss left to recover. For the example project, 10 kW of 400-watt panels needs 4 extra panels to cover the string loss and 3 for microinverters.

Bypass diodes soften the damage, but don't eliminate it

Modern panels include bypass diodes, typically three per module, that let current route around a shaded section instead of blocking the whole panel's output. This prevents a total wipeout from a small shadow, but the panel still loses meaningful output whenever a diode activates. The chimney and vent obstruction loss calculator covers a named rooftop obstruction in more detail, and the output loss from partial shading tool models the annual production impact once your layout is settled.

Trimming the shade beats working around it

Adding panels restores your target output on paper, but trimming a tree or moving an obstruction fixes the underlying cause. If a roof section is permanently shaded, the multi-roof plane array splitter shows how to move panels onto shade-free planes, and the usable roof area calculator confirms the extra panels physically fit.

Questions

Frequently asked

With a standard string inverter, panels are wired in series, so a shaded panel limits the current of the whole string, not just its own output. In one test configuration NREL measured a shadow cutting power by more than 30 times its own size. Typical annual losses are smaller because bypass diodes route current around shaded sections and shade moves through the day, but a string still loses more than the light it blocks.

They recover part of it, not all of it. In NREL's side-by-side test on identical 8 kW arrays, a microinverter system produced 3.7% more energy than a string inverter under light shading, 7.8% more under moderate shading, and 12.3% more under heavy shading. The light physically blocked by an obstruction is still lost either way, since no inverter can generate power from shade.

It depends on how much shade you actually have. NREL's measured gain was about 4% under light shade and about 8% under moderate shade, so the question is whether that extra energy is worth the equipment premium. Compare the gain with your quoted price difference before deciding.

Yes, for a fixed target system size, adding panels to offset a known shading loss is a standard planning approach. It doesn't remove the shade itself, but it can restore your original target output.

Related tools

Keep sizing your system

Usable roof area calculator Multi-roof plane array splitter Roof azimuth orientation calculator DC to AC ratio calculator Solar panel calculator Roof pitch output adjustment calculator Chimney and vent obstruction loss calculator All system sizing tools