Shade and solar in Perth: how to assess if trees and structures will affect your system

Shade is the single biggest performance variable within your control when installing solar. A shaded panel on a standard string inverter doesn't just lose its own output — it can pull down the entire string it's on. Understanding shade, and assessing your specific roof, is one of the most important pre-installation steps.

Why shade hits string inverters hard

In a conventional string inverter system, all the panels in a string are connected in series. The inverter looks at the entire string and optimises based on the weakest link. A single shaded panel forces the rest of the string to operate at a lower power level.

Example: A string of 8 panels, one at 50% output due to afternoon shade.

• Unshaded output: 8 panels × 400W = 3,200W
• With one shaded panel: the shaded panel pulls the string down; effective output approximately 4,000W × (8×7/8) is not how it actually works — the string is effectively limited by the shaded panel's voltage, reducing total output to approximately 2,400–2,800W

This "Christmas lights" effect is why even a small tree casting a shadow on two panels from 2pm onwards can cost several hundred kWh/year.

Systems with shade mitigation:

• Power optimisers (SolarEdge): Each panel has an optimiser that allows it to operate at its individual maximum. One shaded panel doesn't drag down the rest.
• Microinverters (Enphase): Each panel has its own inverter. Complete isolation between panels. Maximum shade tolerance.
• Standard string inverter: No per-panel mitigation. Shade has outsized impact.

Types of shade in Perth

Permanent shade (structures):

• Chimneys, ridge caps, TV antennas, solar hot water cylinders
• Neighbouring walls or structures
• Carports or pergolas

These cast fixed shadows on predictable paths. An installer can map them exactly and design around them.

Seasonal tree shade: Perth trees vary enormously. A deciduous tree may cast shade in winter but not summer. A native eucalypt or peppermint tree casts year-round shade that changes with time as the tree grows.

The peppermint tree question: Perth backyards are full of peppermint trees (Agonis flexuosa) — loved for shade and native character. If a peppermint tree's afternoon shadow reaches your proposed panel array, the financial case becomes a calculation:

• Annual solar generation loss from shade: X kWh/year
• Annual value of lost generation at 33.26c: $Y
• Tree removal cost: $800–$3,000 for a mature peppermint
• Tree pruning cost to reduce shadow: $200–$600 (may need to repeat every 2–3 years)
• Value of tree's cooling effect in summer (reduces AC load): Z kWh/year × 33.26c

The cooling value of a large shade tree in Perth summer is not negligible — a well-positioned mature tree can reduce AC consumption by 500–1,500 kWh/year. In some cases, keeping the tree and accepting reduced solar generation still produces a better overall energy outcome.

How installers assess shade

Visual assessment: The most common method. A competent installer visits at a time that represents a shade-critical moment (early morning or late afternoon in winter when the sun is low) and walks the roof section.

Sun path tools:

• SunSurveyor (iOS/Android app): Uses AR to overlay the sun's path on a camera view of your roof. Shows where the sun will be at any time of day, any day of year.
• Solar Pathfinder: A physical dome device that maps all obstacles by reflecting a 180° view; placed on the roof, it shows shade obstructions across the full year's sun path.
• Shade mapping software: PVsyst, Helioscope, and similar tools used by professional installers can model shade impact on generation and energy yield with detailed 3D modelling.

Red flag: An installer who quotes remotely (photo + satellite imagery only, no site visit) for a property with trees near the roof is giving you a generation estimate with significant uncertainty. Shade assessment requires standing on or near the roof.

Impact of shade on system sizing

If shade is unavoidable on some panels, the system design should route unshaded panels onto strings that avoid the shaded ones, if possible:

• North-facing panels on a clear face → one string (or one MPPT input)
• West-facing panels with afternoon shade from a fence → separate string (or separate MPPT input)

A good hybrid or string inverter with dual MPPT inputs can handle two different-facing strings independently. The shaded string doesn't affect the unshaded one.

Minimum economic threshold: As a rough rule: if more than 10–15% of your proposed panel area receives shade for more than 2 hours per day during summer, it's worth either redesigning the array to exclude those panels, considering power optimisers, or addressing the shade source before installation.

Tree trimming and solar — Perth councils

Perth councils vary in their approach to tree removal on private property. Some native species (particularly Tuart, Jarrah, Marri, Banksia) may have protection orders. Before removing a tree to improve solar access:

• Check with your local council whether the tree requires a permit to remove
• Consider pruning to reduce shadow rather than full removal (cheaper, maintains the tree's value)
• Get a certified arborist assessment if the tree is large or native

Shade impact estimates are illustrative and vary significantly with specific panel and inverter configurations. Professional shade analysis using PVsyst or Helioscope provides more accurate generation estimates for shaded sites.