Solar Panel Shading Analysis: Complete Roof Assessment Guide

The Shading Problem in Plain English

Solar panels need direct sunlight to generate electricity. Any shadow on a panel — from a tree, chimney, neighbouring building, antenna, or even a bird sitting on it — reduces that panel's output. In a conventional string inverter system, a shadow on one panel can reduce the output of all other panels connected in the same string, compounding the problem significantly.

The good news: shading is rarely a binary problem. A heavily shaded roof may still support a worthwhile solar system on unshaded sections. A lightly shaded roof with the right inverter technology can perform nearly as well as an unshaded equivalent. The key is an honest assessment before installation — not an optimistic sales pitch that leaves you disappointed by real-world performance.

Types of Shading and Their Impact

Hard shading (solid shadows): Trees, chimneys, neighbouring buildings, and rooftop structures cast defined shadows. The impact depends on when the shadow falls. A chimney that shades two panels for 30 minutes at 8am has negligible annual impact. A tree that shades half the array for 4 hours during the peak solar production window (10am–2pm) has a major impact.

Soft shading (diffuse light reduction): Haze, thin cloud, and atmospheric dust reduce all panels proportionally. These effects are captured in peak sun hour estimates and don't create the disproportionate string performance problems that hard shading does.

Self-shading: Panels can shade each other if installed with insufficient spacing or too flat an angle on a flat roof. Proper installation avoids this.

The Critical Hours: When Shading Matters Most

Solar energy production isn't distributed evenly through the day. Approximately 75% of daily energy is produced in the 6-hour window centred on solar noon. Shading during this window has roughly 3× the impact of equivalent shading in early morning or late afternoon. When evaluating shade, focus specifically on what shadows fall on your panels between 9am and 3pm.

A tree that casts a shadow during early morning production hours (7–9am) is far less concerning than one that casts a shadow at 12–2pm. Don't let an installer dismiss shade concerns by pointing to morning shadow on an afternoon-heavy roof — or vice versa.

How to Do Your Own Preliminary Shade Analysis

The shadow observation method: On a clear day, observe your roof at 10am, 12pm, and 2pm. Note any shadows falling on the areas where panels would be installed. If the roof is clear of shadows at all three times, shading is unlikely to be a significant issue.

Google Earth/Sun Tools: Google Earth's 3D view lets you estimate nearby building and tree heights. The National Renewable Energy Laboratory's PVWatts tool (pvwatts.nrel.gov) allows you to enter shading percentages by month and calculate system yield impact. It's not a substitute for a professional analysis but gives a useful preliminary estimate.

Solar Pathfinder or Solmetric SunEye: Professional shade analysis tools placed at roof level photograph the entire sky dome and calculate shading factor for every month of the year. A reputable installer should offer this service, sometimes at no charge as part of the quote process.

LIDAR-based shade analysis: Some installers and online services use LIDAR (aerial survey data) to model your roof's exact shade profile using actual tree heights and neighbouring building data. This is the most accurate method and is increasingly available through installer quoting tools.

What Shading Percentage Is Acceptable?

As a rough guide: less than 10% annual shading factor means solar will perform well with a standard string inverter. 10–20% shading factor is the zone where power optimisers (SolarEdge) or microinverters (Enphase) become worthwhile — they'll recover 70–85% of the shading loss that a string inverter would suffer. Above 20% shading factor on a significant portion of the roof, the economics of solar become marginal — you may be better installing a smaller system on the less-shaded sections only.

These percentages should apply to the peak production hours (9am–3pm), not total daylight hours. A 30% shading factor during the first hour of daylight is effectively irrelevant to annual yield.

Inverter Technology Solutions for Shaded Roofs

Standard string inverter: Not suitable for shaded roofs. One shaded panel can drag the entire string to that panel's output level. Avoid on any roof with meaningful shading.

Power optimisers (SolarEdge): Each panel has an optimiser that allows it to operate at its own maximum power point regardless of other panels. Shading one panel affects only that panel's output — all others continue normally. The best solution when budget is a constraint and shading is partial.

Microinverters (Enphase): Each panel operates completely independently with its own AC inverter. Shading one panel has zero impact on any other panel. 25-year warranty matches panels. The premium choice for shaded roofs and multiple roof orientations.

When Solar Genuinely Doesn't Make Sense

If significant shading falls on the only available roof sections during peak production hours and no tree removal or trimming is feasible, solar may not make financial sense. A system producing 50% of its expected yield due to shading has a doubled payback period and may never break even if the shading issue is permanent. An honest installer will tell you this. Be cautious of any installer who dismisses major shading concerns without a quantified analysis.

In these cases, alternatives worth exploring: community solar subscriptions (pay into a shared solar farm, receive bill credits), green power tariffs from electricity retailers, or investing in solar ETFs (ICLN, TAN) as an indirect way to participate in the energy transition if a home system genuinely doesn't make sense for your property.