Performance and output

Output loss from chimney shading calculator

Enter your annual output, the chimney's height above the roof, the sun angle you're planning around, and how many panels sit in its shadow path.

Your system and the chimney

1 Project details Optional
Shown on your exported report. Leave blank to skip.
2 Your system
Use your system's rated annual output with no chimney shading applied.
Find this on a recent utility bill.
3 The chimney and shadow geometry
Measure from the roofline to the top of the chimney, not the ground.
Lower angles, like 20 to 25 degrees, reflect winter or early morning sun and give a conservative estimate.
Count how many panels the chimney's shadow crosses at any point during the day.

How it works

Three steps to a shadow-based loss number

Step 1

Measure the chimney's height above the roof

This is the single biggest driver of shadow length. Unlike a tree covered in the tree shading calculator, a chimney's height is fixed and won't change over time.

Step 2

Choose a conservative sun angle

Using a lower winter sun angle gives a more conservative, worst-case shadow length than assuming summer midday conditions.

Step 3

Count the panels in the shadow path

Even one or two affected panels can register a measurable loss due to bypass diode behavior, covered in more detail in the partial shading tool.

Understanding your chimney shading loss

Why chimneys are usually a minor shading source

A 3 foot chimney casting a shadow across 2 panels at a 25 degree sun angle costs less than 1% of annual output, typically under 50 kWh a year for a 10,800 kWh/year system. Chimneys are usually a minor shading source compared to trees or neighboring buildings, since their compact footprint limits how many panels they can actually reach. A NLR study on partially shaded PV arrays found that shading covering just half of one cell can eliminate the output of an entire 18-cell bypass diode group, showing how a narrow obstruction like a chimney can cost more than its silhouette suggests.

The real geometry behind shadow length

Shadow length follows basic geometry: height divided by the tangent of the sun's elevation angle. A lower sun angle, common in winter or early morning, stretches a short chimney's shadow much further across the roof than the same chimney would cast at midday in summer. This is why the same 3 foot chimney can cast a 6 foot shadow at a 25 degree sun angle but an 11 foot shadow at 15 degrees. NOAA's Solar Calculator can work out the sun's elevation angle for your exact location, date, and time if you want a more precise shadow length than a rough seasonal estimate. Check the seasonal shading variation calculator if you want to see this same effect applied across a full year.

Why even a small chimney shadow can register a measurable loss

Yes, if even a corner of a panel falls in its shadow path, since bypass diode behavior can shut down a larger section of that panel's output than the shadow's physical size would suggest. The panel count in the shadow path matters more than the chimney's overall size, which is why this tool asks for that figure directly. This same bypass diode behavior is exactly why equipment choice matters here too, since the microinverter vs string inverter output tool shows how a shaded panel's loss stays contained to that one panel with microinverters instead of dragging down its whole string.

Whether it's worth planning panel layout around the chimney

Often yes, since shifting one or two panels away from a chimney's known shadow path during layout design is usually far cheaper than losing that output for the system's entire lifetime. This is a common adjustment installers make during the design phase, well before any panels go on the roof. The roof inspection cost before solar estimator prices the roof check that comes before any of that loss is built.

What to do with this number next

If your roof has multiple obstructions, run the general shading loss estimator for a combined estimate, or check the neighboring building calculator if a nearby structure is also part of the picture.

Questions

Frequently asked

A 3 foot chimney casting a shadow across 2 panels at a 25 degree sun angle costs less than 1% of annual output, typically under 50 kWh a year for a 10,800 kWh/year system. Chimneys are usually a minor shading source compared to trees or neighboring buildings.

Shadow length follows basic geometry: height divided by the tangent of the sun's elevation angle. A lower sun angle, common in winter or early morning, stretches a short chimney's shadow much further across the roof than the same chimney would cast at midday in summer.

Yes, if even a corner of a panel falls in its shadow path, since bypass diode behavior can shut down a larger section of that panel's output than the shadow's physical size would suggest. The panel count in the shadow path matters more than the chimney's overall size.

Often yes, since shifting one or two panels away from a chimney's known shadow path during layout design is usually far cheaper than losing that output for the system's entire lifetime. This is a common adjustment installers make during the design phase.

Related tools

Keep exploring your output

Output loss from tree shading Output loss from partial shading Output loss from neighboring building Shading loss estimator Morning vs afternoon shading impact Seasonal shading variation Real world vs nameplate output gap All performance and output tools