Solar Panels and Shade: Trees, Chimneys & Neighbors

On a string-inverter system, panels are wired in series, and every panel in a string carries the same current. The same thing happens inside each panel: in NREL’s description of a typical 60-cell module, the cells form three 20-cell sections, each protected by its own bypass diode (NREL, 2012). When shade keeps a few cells in a section from carrying the string’s current, the diode routes the current around that whole section — so a shadow on a few cells can take a third of the panel offline while the rest of that section sits in full sun. NREL notes that in some layouts a shadow blocking as little as 25% of the light on a single cell can be enough to start a bypass diode conducting. Bypass diodes keep one shaded section from throttling the whole string, but they don’t make shade free.

That gives shade two costs: the direct loss of the light it blocks, which no electronics can recover, and the mismatch loss it forces on sunlit cells and panels wired with the shaded ones. The second part is what module-level electronics are built to win back. For the basics of cells, strings and inverters, see how solar panels work.

A microinverter under each panel, or a DC optimizer on each panel feeding a string inverter, keeps a shaded panel from setting the pace for its neighbors. In NREL’s side-by-side test both did about the same — and both recovered only part of the loss:

With module-level electronics the arrays produced about 2%, 5% and 8% more energy than the string-inverter reference under light, moderate and heavy shade. Across NREL’s trials the devices recovered 25–35% of annual shading losses, and the spread came mostly from how many parallel strings the test array had rather than from the brand (NREL, 2016). For estimates, NREL suggests assuming they recover about a third of the shading loss a site survey measures, within a range of 25–40% (NREL, 2015). So a roof face with a measured 12% annual shading loss would be modeled at about 8% with microinverters or optimizers (12% × [1 − 0.33]).

They help in two quieter ways as well. NREL expects them to eliminate the small mismatch between panels that exists even without shade — PVWatts budgets 2% for it by default — and they report every panel’s output, so a new shadow or a failing module shows up in the app. What they can’t do is make light: a panel in deep shade still produces little. For the two main platforms side by side, see Enphase vs SolarEdge.

Read it as a multiplier. A tree whose crown rises 20 feet above your panels casts a noon shadow about 31 feet long in late December (20 × 1.57), about 13 feet at the equinoxes and under 4 feet in late June. Shadows point away from the sun — north-northwest at 10 a.m. and north-northeast at 2 p.m. in winter — so the long winter shadows come from whatever stands to the south, southeast and southwest of the array; in summer, trees to the east and west matter early and late in the day. California’s energy code makes the same point from the other side: obstructions north of every panel need not be counted as shading at all (JA11.3). How much the winter dip matters depends on the rest of the year — see solar panels in winter and Los Angeles sun-hour data.

Annual solar access is the sunlight a spot receives over a year with shading, divided by what it would receive with no shading. California’s 2025 Energy Code uses it for new homes in two ways: roof areas below 70% annual solar access are left out of the solar access roof area that can cap the size of the required system (§150.1(c)14), and on the prescriptive path the finished array generally has to average at least 98%, weighted by panel count (JA11.3.1). The code also requires the check to be done with a CEC-certified solar assessment tool that reports annual solar access for each array, and says tools working from satellite or aerial images must give values comparable to on-site measurements (JA11.4).

Those rules are written for new construction, and a system added to an existing home generally isn’t held to them — but they are a good template for what to ask any installer for: a shade report with annual solar access for each array. The measuring ranges from a sun-path tool such as the Solar Pathfinder, which NREL’s solar-ready buildings guide suggests for reading the sky view at a panel location, to software that models shade from aerial data — Google’s Solar API, for one, publishes a digital surface model, annual and monthly flux maps and hourly shade layers. If a production estimate simply uses PVWatts’ default 3% shading loss, nobody has measured your roof yet. Orientation matters as much as shade when choosing faces — see which way solar panels should face.

Trimming is the usual first move: often you only need to clear the branches that cross the sun’s path during the hours that matter, and in winter that path runs low across the southern sky. Trees grow back, so plan on trimming as ongoing maintenance. Shade tends to creep up over the years: in a study of 503 systems, home rooftops with medium or heavy shade lost energy yield at a median 2.1% a year, against 1.3% for home rooftops overall — a whole-system figure that includes growing foliage, dirt and inverter losses (Deceglie et al.).

Removal is permanent and can clear a whole roof face, but weigh what the tree does for you. California’s own shade law opens by saying the state encourages trees and shrubs that create shading, moderate outdoor temperatures and provide economic and aesthetic benefits (PRC §25980). Permits matter too. In the City of Los Angeles, a permit from the Board of Public Works, issued through StreetsLA’s Urban Forestry Division, is required before anyone removes, trims or root-prunes a street tree in the public right-of-way, and before anyone removes or relocates a native protected tree or shrub on private land: native oaks (except scrub oak), Southern California black walnut, western sycamore, California bay, toyon and Mexican elderberry, once they measure 4 inches or more in cumulative diameter 4½ feet above the ground. “Removal” includes anything that would cause the tree to die, such as damaging its roots (LADBS). Other cities and HOAs have their own rules.

A neighbor’s tree is a separate matter. Cutting it without agreement can be expensive: California law sets damages for wrongful injury to trees on someone else’s land at twice or three times the harm (Civil Code §3346). Often the best answer is a mix — trim what you can, with the owner’s agreement, design the array around what’s left, and let module-level electronics absorb the rest.

The Solar Shade Control Act (Public Resources Code §§25980–25986) says that once a solar collector is installed, the owner of another property may not allow a tree or shrub to be placed, or once placed to grow, so that it casts a shadow over more than 10% of the collector’s absorption area at any one time between 10 a.m. and 2 p.m. local standard time (§25982) — 11 a.m. to 3 p.m. on the clock during daylight saving time. A tree kept in violation is a private nuisance if its owner fails to remove or alter it after receiving written notice from the collector’s owner (§25983). Until 2009 a violation was a public nuisance and an infraction, with a fine of up to $1,000 per violation; SB 1399 (2008) replaced that with today’s private-nuisance route and added several of the exemptions below, including the one for trees planted before the panels.

The details decide whether it applies at all. A “solar collector” under the Act is a system on the roof of a building; a ground-mounted system counts only if the roof can’t take it because of roofing material, slope, structural shading or orientation, and a system designed to offset more than the building’s electricity demand doesn’t count. The collector must also meet local building and setback rules and sit at least 5 feet from the property line and 10 feet above the ground, or lower only with an extra setback of three times the amount lowered (§25981). Before installing, you may send neighbors a formal Solar Shade Control Notice by certified mail, no more than 60 days ahead, using wording set out in the statute (§25982.1).

The Act does not apply to:

• Trees or shrubs planted before your collector was installed.
• Trees on timberland or on land producing commercial agricultural crops.
• A replacement for a tree or shrub that was there before your panels and later died or was removed for public health, safety or environmental reasons.
• A tree or shrub subject to a city or county ordinance — in Los Angeles that may include street trees and protected native trees.
• Buildings, additions and other structures: the Act covers trees and shrubs only.

A city, or a county for unincorporated areas, can also adopt an ordinance exempting its whole jurisdiction, and a local tree-preservation or solar-shade ordinance governs where one exists (§25985). Because of those carve-outs, the Act fits a fairly narrow set of cases. It is worth knowing, but it is no substitute for a good design.

A chimney, plumbing vent, attic vent, dormer or satellite dish casts a small shadow that sweeps across nearby panels through the day, and California’s energy code counts all of them as shading obstructions for new homes. The standard fixes are layout and electronics: set panels back so the moving shadow clips as few modules as possible, put panels on roof faces the obstruction doesn’t reach, and use module-level electronics so any panel that does get clipped drops on its own. Occasionally a vent or dish can be relocated — but vents tie into plumbing and attic ventilation, so that is a call for the installer to assess against the code and your roof, not a default move. In practice, a careful layout handles most of it without moving anything.

Shade is real, but it’s rarely a deal-breaker. Measure it for each roof face, design around it with placement and module-level electronics, leave faces empty that don’t earn their keep, and treat tree trimming as maintenance — leaning on the Solar Shade Control Act only in the narrow cases it fits. Get the design right and a partly shaded roof can still be a strong solar roof. Not sure how much sunny roof you have? Start with how many solar panels you need in California, and we’ll size a system around your real sun.

Figures on this page come from the primary sources below and, where noted, from Cali Energy calculations using the stated assumptions. Rates, incentives, and program terms change; each was verified September 30, 2026.

1. California Legislative Information — Solar Shade Control Act, Public Resources Code §§25980–25986
2. California Legislature — SB 1399 (2008), Chapter 176: chaptered text and Legislative Counsel’s Digest
3. California Legislative Information — Civil Code §3346 (damages for injury to trees)
4. NREL — Photovoltaic Shading Testbed for Module-Level Power Electronics (NREL/TP-5200-54876, 2012)
5. NREL — Photovoltaic Shading Testbed for Module-Level Power Electronics: 2016 Performance Data Update (NREL/TP-5J00-62471)
6. NREL — Modeling Microinverters and DC Power Optimizers in PVWatts (NREL/TP-5J00-63463, 2015)
7. NREL — PVWatts Version 5 Manual (NREL/TP-6A20-62641)
8. Deceglie et al. — Fleet-Scale Energy-Yield Degradation Analysis of Residential and Nonresidential PV Systems
9. California Energy Commission — 2025 Building Energy Efficiency Standards, Title 24 Part 6 (CEC-400-2025-010-F)
10. California Energy Commission — 2025 Reference Appendices, Joint Appendix JA11 (CEC-400-2025-010-AP)
11. NOAA Global Monitoring Laboratory — Solar Calculator: calculation details
12. NREL — Solar Ready Buildings Planning Guide (NREL/TP-7A2-46078)
13. Google Maps Platform — Solar API overview
14. LADBS — Information Bulletin P/ZC 2024-022: Guidelines and Requirements for Regulated Trees