How a solar shade analysis measures the sunlight your roof actually gets

If a chimney, a neighbor's oak, or a plumbing vent throws a shadow across your roof, you need to know what it costs before you buy panels. A shade analysis (also called a sun or solar access study) answers that question, and the answer is almost never "a little." Shading interacts with how solar cells are wired in ways that punish output far more than the shadow's size would suggest.

Why shade destroys production nonlinearly

Solar cells inside a module are wired in series into strings, and modules themselves are often wired in series too. In a series circuit, current is limited by the weakest link. When one cell is shaded, it stops producing current and behaves like a resistor, choking the current for every other cell in that series path. This is the core reason shade is nonlinear: cover 5% of a module and you can lose far more than 5% of its output.

Bypass diodes soften this. Most modules split their cells into two or three sub-strings, each with a diode that lets current route around a shaded section. That prevents the worst-case total shutdown and stops hot-spot damage, but it comes at a cost: when a diode activates, that entire sub-string, a third or half the panel, drops out. So partial shade on one module can knock out a large chunk of it, and in a series string of modules, that pulls down every panel wired with it. This string effect is why installers care about shade patterns, not just shade totals. Our deeper explainer on shading and solar production walks through the diode behavior in detail.

What a shade analysis actually measures

The goal is solar access: the percentage of available annual sunlight a spot on the roof receives after obstructions are accounted for. A good analysis captures three things:

• The horizon and skyline — trees, hills, adjacent buildings, and rooflines that block low-angle morning and evening sun.
• On-roof obstructions — chimneys, vents, dormers, satellite dishes, and HVAC units that cast moving shadows through the day.
• Seasonal sun paths — the sun sits low in winter and high in summer, so an obstruction that's harmless in July can dominate December.

The output is usually a solar access percentage per location, or a monthly breakdown, that feeds directly into a production model. Roof orientation matters alongside shade; see solar panel tilt and azimuth for how those combine.

Satellite sun-hour heat-maps

Satellite-derived flux maps estimate how much sunlight each pixel of a roof receives over a year, expressed in kilowatt-hours per kilowatt of panel per year (kWh/kW/yr). These models trace the sun's path across the sky for every day of the year and test whether nearby geometry, derived from 3D building and terrain data, blocks it. The result is a heat-map: bright zones get full sun, dark zones near the shaded eave or the north side of a chimney get less.

This is fast, remote, and free of a site visit, which makes it ideal for early screening. Our free instant estimate uses this kind of per-roof sunlight model to flag good and bad roof planes from just an address. The limits are real: satellite imagery can predate a tree's growth or a neighbor's new addition, and the modeled tree canopy is an approximation, not a survey. Treat a heat-map as a strong first pass to confirm, not a final verdict.

On-site tools: drone, Solar Pathfinder, SunEye

When the money is on the line, installers verify shade on the roof itself.

• Solar Pathfinder — a domed reflector that mirrors the whole sky onto a sun-path chart. In one photo you see every obstruction's outline overlaid on the sun's seasonal arcs, giving a monthly shade reading with no batteries and no software required.
• Solmetric SunEye and phone-based apps — a fisheye camera captures the skyline, and software overlays the sun path to compute solar access percentages per month automatically.
• Drone flights — a drone photographs the roof and surroundings, and photogrammetry builds a 3D model that simulates shadows across the year. Useful for large or complex roofs where standing on every plane is impractical.

Each captures the actual horizon as it exists today, catching the neighbor's tree the satellite model missed.

How module-level electronics mitigate shade

You can't remove a shadow, but you can stop it from spreading. Microinverters and DC power optimizers operate each module independently, so a shaded panel drops only its own output instead of dragging its whole series string down with it. On an unshaded roof the benefit is modest, but on a roof with scattered shade it can meaningfully raise annual production and give you per-panel monitoring. Compare the approaches in microinverters vs string vs optimizers and the focused what is a power optimizer guide.

Module-level electronics are a mitigation, not a cure. If a roof plane sits under heavy canopy for six hours a day, no electronics will make it productive; the honest move is often to skip that plane. A shade analysis is what tells you which planes are worth wiring and which to leave bare, before hardware is ever ordered.

Putting it together

Run the remote heat-map first to screen the roof, then verify the questionable planes on-site, then choose an inverter architecture that fits the shade pattern you found. That sequence keeps you from over-paying for optimizers you don't need or, worse, filling a shaded plane with panels that never earn back their cost. Start with the free instant estimate to see your roof's sun-hour map, and if the roof looks marginal, read is my roof good for solar before going further.

Educational reference, reviewed 2026-07. A design aid, not a substitute for a licensed electrician or PE. Confirm the enforced NEC edition and local amendments with your AHJ.