Solar shade analysis for Perth homes: trees, chimneys, and neighbours

Shading is one of the most consequential variables in Perth solar performance — and one of the least transparent in some installer quotes. A shaded panel doesn't just lose its own output; on a standard string inverter, it can drag down the output of every other panel in the same string. Here's how to assess shading before signing a contract.

Why shade matters more than you might think

On a standard string inverter, all panels in a string are connected in series. The string's current is limited by the panel producing the least current — typically the most shaded one. A single panel with 50% shade can reduce the entire string's output by 30–50%.

This is the "weakest link" problem. It's why:

• North-facing roofs with no shade genuinely outperform north-facing roofs with partial afternoon tree shade
• The difference between a shaded and unshaded installation isn't linear — partial shade can have outsized impacts

Mitigations for shaded roofs:

• DC power optimisers (SolarEdge) allow each panel's output to be optimised independently — one shaded panel doesn't drag down others in the string
• Microinverters (Enphase) achieve complete independence — each panel operates at its own maximum regardless of adjacent panels
• Avoiding the shaded area entirely — sometimes the right answer is fewer panels on unshaded roof sections rather than more panels with partial shade

Common shading sources in Perth

Trees: Perth's suburban trees — marri, jarrah, melaleuca, peppermint — can cast significant shade on north-facing roofs, especially in winter when the sun is lower. A 5m peppermint tree on the south or south-east of a property doesn't shade in summer but may shade the morning section of the roof in June–August when the sun rises more to the north-east.

Chimneys and vents: Old Perth homes often have brick chimneys that cast a moving shadow across the roof. A chimney 600mm tall on a low-pitch roof casts a shadow that traverses the roof array multiple times per day. Even a small shadow moving across panels for 2 hours can meaningfully reduce annual output.

Roof ridges and hips: On hip roofs (common in Perth), the hip edge creates a shadow on lower panels on adjacent roof faces. Panels near the hip edge of a west-facing section may be shaded by the ridge in the morning.

Neighbouring buildings: In Perth's older suburbs with smaller blocks, two-storey additions can shade a neighbour's north-facing roof. Consider whether adjacent properties have planned or permitted two-storey development.

Parapet walls and HVAC equipment: Commercial buildings and some newer residential units have parapets or rooftop AC units that cast shade in certain sun angles.

DIY shade assessment tools

Several free or low-cost tools help Perth homeowners assess shade before consulting installers:

SunSurveyor (mobile app): Uses your phone's camera with an AR sun path overlay. Point at your roof and see where the sun will be at 9am, 12pm, 3pm for different months. Shows clearly which obstructions cast shade across the panels at different times of year. Available for iOS and Android.

SolTrace / PVWatts (NREL online tool): Enter your address, system size, and roof orientation. PVWatts estimates annual generation but doesn't account for specific local obstructions — it models clear-sky output. Useful as a no-shade baseline to compare against.

The shadow test: On a clear day in winter (June), observe your north-facing roof at 9am, 12pm, and 3pm. Where are shadows falling? Note that winter shade is the worst case — if the roof is shade-free in winter, it will be shade-free year-round.

Google Maps winter sun angle: Google Earth Pro (free) has a sun position slider — drag it to December vs June and observe shadow direction.

What to ask your installer about shading

When getting quotes, ask installers:

1. "Have you assessed shading from [specific tree/structure]?" — An experienced installer will note obstructions in their proposal. If they don't mention a visible 6m peppermint on the property, ask directly.

2. "What's your assumed performance loss from shading?" — If the installer's model assumes no shading loss but there's a tree on the property, the quoted savings figure is optimistic.

3. "Should we use power optimisers or microinverters given the shading?" — If there's partial shade, ask whether the additional cost of shade mitigation technology is justified by the performance improvement.

4. "What happens if a panel is shaded in your system design?" — This reveals whether they've designed for the string-drag problem or planned around it.

Tree management and solar

Perth homeowners often consider removing trees to maximise solar performance. Some considerations:

Council tree permits: Trees >3m on private land in many Perth councils require a permit to remove. Cities including Subiaco, Fremantle, Vincent, Cambridge, and the Town of Victoria Park have significant tree protection requirements. Check with your local council before assuming a tree can be removed.

Tree trimming vs removal: Trimming to reduce canopy on the north face of a tree (the side casting shade onto the roof) is often possible without removal and may require a permit rather than full council approval.

Native species protections: Marri and jarrah trees may have additional protections in some local planning schemes.

Alternative: design around the tree: Sometimes the better outcome is avoiding the shaded roof section entirely and installing panels on an unshaded east-west split, with microinverters or DC optimisers to maintain per-panel performance on that configuration.

Shade impact on system ROI

A rough rule of thumb: 1 hour of significant shading (>50% of panels, >30% coverage) per day reduces annual output by approximately 5–8% relative to an unshaded system. Significant shade for 3–4 hours per day can reduce output 20–30%.

The economics of shade mitigation technology:

• SolarEdge power optimisers add ~$800–$1,500 to a 10kW system cost
• If shading would otherwise reduce output by 20%, a 10kW system generating 14,000 kWh/year loses ~2,800 kWh/year — at A1 retail value of ~$930/year. Payback on the optimisers is roughly 1–1.5 years.
• In a high-shade scenario, power optimisers almost always pay for themselves.

Shade from trees and roof structures is the most consequential variable in Perth solar performance that installer quotes don't always address explicitly. Check shading in winter (the worst case), ask installers how they've accounted for obstructions, and consider shade-mitigation technology (SolarEdge or Enphase) before signing a contract on a partially shaded roof.