Understanding solar shading losses
How much production partial shading typically costs
A 150 kW system that would produce about 215,496 kWh a year unshaded loses roughly 11,314 kWh if 15% of the array is affected by shading that cuts production in that section by 35%, an overall loss of about 5.3% across the whole system. The loss is concentrated in the shaded portion, not spread evenly, which is why even a modest shaded area can matter more than it first appears. NREL's own PVWatts shading loss documentation covers how this concentrated-loss effect is modeled, and use the adjusted production figure in the commercial solar payback calculator for a realistic payback estimate.
Why shading on one section can affect more than that section
Depending on the electrical design, panels wired together in a string can all be limited by the output of the most-shaded panel in that string, a phenomenon sometimes called the Christmas-lights effect. Microinverters or power optimizers on individual panels can reduce this string-level penalty, a difference a DOE-funded NREL shading test measured directly, finding microinverters improved production by 3.7% to 12.3% over string inverters depending on shade severity, so the electrical design matters as much as the physical shading itself, a factor worth discussing alongside the system size by square footage tool when planning the electrical layout.
What typically causes shading on a commercial roof
Rooftop HVAC units, exhaust stacks, and parapet walls are common on-roof sources, while nearby trees, taller adjacent buildings, and utility infrastructure are common off-roof sources. A professional shading analysis using tools like a solar site survey instrument can map these precisely before a system is designed, often as part of the same visit covered by the roof structural assessment cost tool. Our solar and roofing integration section covers how those same on-roof obstructions interact with roofing material choices long-term. Incentives change the budget a lot. The federal tax credit calculator in our Rebates & Incentives section can help you estimate the federal credit and rebates before you commit.
Whether shading losses can be fixed after installation
Sometimes, if the shading source can be trimmed or removed, like overgrown trees, or if the electrical design can be upgraded to isolate the shaded panels' impact. If the shading source is a permanent structure like an adjacent building, redesigning the layout to avoid that area in the first place is usually more cost-effective than a fix after installation, which is worth weighing against the roof mount vs. ground mount cost tool if the roof is heavily obstructed, or the roof space vs. output estimator to see how a smaller unshaded footprint compares. If demand charges are a big part of your bill, the backup runtime estimator in our Battery & Storage section can show whether a battery would shave your peak enough to pay for itself.