What this calculator answers

Residential solar panels lose far more output to partial shading than most homeowners expect. A single shaded panel can drag down the output of an entire string of panels because of how the electricity flows in series. This tool estimates the annual dollar cost of shading from trees, neighboring buildings, chimneys, or vent stacks, and projects that cost over the 25 year typical lifespan of a panel array.

How the math works

Start with the array's rated output in kilowatts, multiply by the local solar productivity factor (your annual kilowatt-hours produced per installed kilowatt, typically 1,100 to 1,700 depending on region), and multiply by your electricity rate to get the unshaded annual production value. Apply the shading loss percentage (typically 5 to 25 percent for partial shading) to get the annual cost of shading. Multiply by 25 years (the typical panel warranty period) and adjust for expected electricity rate inflation to get the lifetime cost.

When to use it

• You are quoting solar installations and want to compare two roof areas with different shade profiles.
• You are deciding whether to trim or remove a tree that partially shades your existing array.
• You are evaluating microinverters or DC optimizers (which mitigate shading losses) versus a traditional string inverter and want to quantify the benefit.
• You are buying a home with existing solar and want to know how much shading from nearby growth is costing the system.

Common mistakes

• Underestimating string inverter losses. Without microinverters or optimizers, a single shaded panel can knock 30 to 70 percent off a whole string's output, far more than the shaded panel's share alone.
• Forgetting that shade patterns change with season. Winter sun angles are much lower, casting longer shadows from the same trees and buildings.
• Ignoring electricity rate inflation. Residential rates have risen about 2 to 4 percent annually over the past two decades. Lifetime shading cost calculated at today's rates understates the real number.
• Confusing module efficiency loss with shading loss. Even unshaded panels lose about 0.5 percent of output per year due to degradation. Shading loss is on top of that.

A worked example

You have a 10 kilowatt solar array in a region with 1,400 kilowatt-hours per kilowatt per year of solar productivity, producing 14,000 kilowatt-hours annually at full output. Your retail electricity rate is 18 cents per kilowatt-hour, making unshaded production worth 2,520 dollars per year. A nearby chimney shades two panels for several hours each afternoon, causing an estimated 12 percent total array loss. That is 1,680 kilowatt-hours of lost production per year, worth about 302 dollars annually or roughly 9,400 dollars over 25 years assuming 3 percent annual rate inflation.

Frequently asked questions