How Shading Affects Solar Panels and How to Reduce Losses

A small shadow can pull a whole panel down. The cause is electrical, not only lost light. This guide explains how it happens, what bypass diodes and optimizers recover, and how to estimate the loss on a real roof.

Last updated: November 2026

A chimney shadow, a tree and the row in front each hurt output in a different way. For the full site workflow, see the solar shading analysis guide.

Why can a small shadow cost so much power?

Two things happen when part of a panel is in shade.

1. Less sunlight. Shaded cells get less light. Some diffuse (scattered) light still reaches them.
2. Electrical mismatch. Cells in sun and cells in shade work at different current and voltage. When they share a circuit, they cannot all run at their best point.

In a string, the panels are wired in series, so they share one current. The weakest part limits the rest. That is why a small shadow can cost more than its size suggests.

NREL’s 2009 field test used a string of ten crystalline silicon panels. Each panel had 3 groups of 18 cells, and each group had its own bypass diode. The paper reports these results:

• Fully shading one cell made its bypass diode switch on. The panel’s voltage, and so its power, dropped by one third.
• Shading more than 40% of one cell took out the power of its whole 18-cell group.
• A shadow over half of one cell cost as much as losing 36 times that area. The paper calls this a worst case.
• The paper says a shadow can cost over 30 times its own size in this kind of setup.

These are results for one test string. Other panels and wiring give other results. The test shows the pattern: small, well-placed shade can cost a lot.

Does one shaded cell shut down the whole array?

Not in every case. In the NREL test, one fully shaded cell cut that panel’s voltage by one third, and the string kept running. A bypass diode limits the damage to the group of cells it protects.

Also keep power and energy apart. Power is measured in watts (W) at one moment. Energy is power over time, in kilowatt-hours (kWh). A short, heavy shadow and a light shadow that lasts all summer can cost the same energy.

What do bypass diodes do, and what can’t they do?

A bypass diode gives current a way around a group of shaded cells. That keeps the shaded cells from overheating, and it lets the rest of the panel keep working. NREL’s paper says protection from reverse bias on shaded cells is the diodes’ main job. Better output in shade is a useful extra.

A diode cannot bring the sunlight back. When it switches on, the panel loses the voltage of that whole group of cells. The group makes no power.

The inverter also matters. PVsyst’s help page on bypass diodes (PVsyst is a solar simulation program) explains two limits:

• A shaded string runs at a lower voltage. If that voltage falls below the inverter’s minimum MPPT voltage, the loss rises fast. MPPT (maximum power point tracking) is how the inverter finds the best working point.
• The more strings share one MPPT input, the worse the loss. In its example, three strings on one input lose all direct-sun output when one third of the sub-modules are shaded. Only the diffuse light remains. The example assumes 15% diffuse light and shade along the bottom row of a row layout.

So “one group is bypassed, so I lose one third” is only true in simple cases.

Do half-cut panels handle shade better?

Sometimes. A half-cut panel has cells cut in two. The shade result depends on how the panel is wired inside. It does not depend on the name.

PVsyst’s page on module layouts describes the common twin half-cut design. Two halves of the panel are wired side by side, and they share one bypass diode. If shade covers the bottom part, the upper part keeps working. PVsyst says these panels suit portrait mounting in row layouts.

So a half-cut panel does not automatically have twice as many bypass diodes. Check the panel’s datasheet for the real circuit. For more, see half-cut vs full-cell solar panels.

Do optimizers and microinverters fix shading?

They help, but they do not remove it. The U.S. Department of Energy’s inverter primer explains the choice:

• A string inverter converts the power of a whole string. If one panel is shaded, the string makes less power.
• A microinverter sits on each panel. Shade on one panel does not change the power you draw from the others. Microinverters can cost more.

A DC optimizer is a small device on each panel that adjusts its voltage and current. Optimizers and microinverters are called module-level power electronics (MLPE). See the MLPE glossary entry.

NREL measured how much they help. A 2014 conference paper looked at 542 systems with module-level optimizers, in 26 countries. Most (404) were under 10 kW. The data came from one optimizer maker’s database (Tigo Energy). The paper found:

The authors warn that these systems may not be typical. Owners may add optimizers on roofs with more shade than average. So treat 8.3% and 13% as examples. They are not a promise for your roof.

An optimizer does not make a shaded panel sunny. It only stops that panel from dragging down the others. A PVsyst study presented at the Sandia PV Performance Modeling Workshop agrees. It says optimizers reduce mismatch loss, and that some types work better for some layouts. It also says the inverter and optimizer current and voltage limits matter.

Which solar panel shading solutions work?

Try the cheapest fix first. The table lists common fixes in that order.

The sources above give no loss figure for each fix, so the table has none. The only sourced number is the optimizer result above. A site model gives the figure for your roof.

What should you record about each shade source?

Write down the shade pattern before you estimate any loss.

A photo taken at noon in winter is not a yearly assessment. Morning and evening shade differ. So do seasons and tree leaves. A shade-free summer visit does not prove a shade-free year.

How do you estimate yearly energy loss?

Ask the designer to state:

• The weather data, the years it covers and the time step.
• How the obstruction survey was done and how exact it is.
• The panel model, its direction and its wiring.
• The inverter or optimizer limits in the model.
• Whether the result splits lost sunlight from electrical mismatch.
• What changes between the layouts you compare.

A shadow picture shows where light is blocked. It is not a yearly loss result by itself. You need a model that includes the wiring. SurgePV’s shading analysis helps with the shade part of this work. You can see it in a free demo. For the wider workflow, read the shading analysis hub.

Worked example with the NREL medians

This is an illustration, not a measurement. Say a roof would make 10,000 kWh a year with no shade. Use the NREL medians from the table above, including the estimated 13%.

The arithmetic: 10,000 x 0.13 = 1,300. 10,000 x 0.083 = 830. 1,300 - 830 = 470. And 470 / 1,300 = 36%, the share NREL reports as recovered.

Your own roof can land well above or below these numbers. Price the extra 470 kWh against the cost of the optimizers before you decide. The numbers in this example are illustrative, built from the NREL medians above.

Want to see this on your own layout? Book a demo and bring a roof plan. To explain shade loss to a customer, read how to explain solar shading losses to homeowners. For wiring after shade is mapped, see solar string design mistakes. Related terms: shading analysis and bypass diode.

Frequently Asked Questions

How much does shade reduce solar panel output?

It depends on how the shadow crosses the cell groups, how the panels are wired, and when the shade falls. In NREL’s field test, shading one cell cut that panel’s power by one third. In NREL’s study of 542 systems with optimizers, the median yearly shade loss was 8.3%. The same systems would have lost 13% with plain bypass diodes.

Do solar panels still work in partial shade?

Yes. Unshaded cells keep making power, and diffuse light still reaches shaded cells. A bypass diode lets current go around a shaded group of cells. The panel then makes less power, because that group adds nothing.

What is the best inverter for a shaded roof?

There is no one answer. Model your layout with a string inverter and with module-level devices, using the same site data. Optimizers and microinverters cut mismatch loss, and they cost more. Whether they pay off depends on how much shade you have.

How do you assess shading before installing solar panels?

Record where each obstruction is and how tall it is. Note the panel positions, direction and wiring. Then use a model that covers sun position across the year and the electrical effects. One photo of one shadow is not enough.

Do half-cut panels always perform better in shade?

No. A common half-cut design wires two halves side by side with one shared bypass diode. That helps when shade covers only one half. The result depends on the panel’s real circuit and on how you mount it. Check the datasheet. Do not assume a half-cut panel has six bypass diodes.

Can shade damage a solar panel?

It can, which is why panels have bypass diodes. A bypass diode gives current a path around a shaded group of cells, which keeps those cells from overheating. NREL’s paper says protection from reverse bias on shaded cells is the diodes’ main job.