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

Wondering if that gum tree, chimney, or the neighbour's two-storey addition will hurt your solar output? Check your roof for shadows at 9am, noon, and 3pm on a clear day in June. Winter is the worst case for shade in Perth, so if the roof is clear then, it stays clear the rest of the year. That one test tells you more than most installer proposals will.

Shading is one of the most consequential variables in Perth solar performance, and one of the least visible in a sales pitch. On a standard string inverter, panels are wired in series, so a single shaded panel can drag down the output of every other panel in the same string, not just its own share of the roof. Here's what actually casts shade on a Perth home, how to check for it yourself, and what to ask before you sign.

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Why does shade matter more than one shaded panel?

Weakest-link wiring: on a standard string inverter, every panel in a string is connected in series, so the whole string's current is capped by whatever panel produces the least, usually the most shaded one. Cover a single panel and the string can lose more than that one panel's own share of output.

It isn't linear either. A north-facing roof with genuinely no shade will outperform a north-facing roof with partial afternoon tree shade by more than the shaded hours alone would suggest.

Ways to fight the string-drag problem:

• DC power optimisers (SolarEdge is the common brand) let each panel's output be tuned independently, so one shaded panel stops dragging down the rest of the string.
• Microinverters (Enphase is the best-known brand) go further again: each panel runs at its own maximum, fully independent of its neighbours.
• Working around the shade is sometimes the better call: fewer panels on the unshaded roof section beats more panels split across shaded and clear ground.

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What actually casts shade on a Perth roof?

Trees, chimneys, roof geometry, and the house next door are the usual culprits, and Perth's older, tighter-block suburbs make all four more common than newer estates.

Trees. Perth's suburban trees, marri, jarrah, melaleuca, peppermint, throw real shade on north-facing roofs, especially in winter when the sun sits lower. A peppermint on the south or south-east side of a block won't touch the roof in summer, but come June to August, when the sun rises further to the north-east, it can shade the morning section.

Chimneys and vents. Older Perth homes often carry a brick chimney, and its shadow moves across the roof through the day. Even a modest chimney on a low-pitch roof can sweep shade across the array more than once, and just a couple of hours of that moving shadow can meaningfully dent annual output.

Roof ridges and hips. Hip roofs, common across Perth, throw their own shadow onto lower panels on the adjacent face. A west-facing section near the hip edge can sit in the ridge's shadow through the morning.

The house next door. Perth's older suburbs sit on smaller blocks, and a neighbour's two-storey addition can shade a north-facing roof in ways a single-storey home never would. If a two-storey build is planned or already approved next door, factor it in before you commit to panel placement. If a neighbour's tree or structure is already shading an existing system, see our guide to what you can actually do about a neighbour's tree or building shading your solar, since there's no automatic right to sunlight in WA, though new developments still have to preserve some solar access under the state's planning rules (State Planning Policy 7.3 and the Residential Design Codes) where the neighbouring block is zoned for lower-density housing.

Parapets and rooftop plant. Commercial buildings, and some newer residential units, carry parapet walls or rooftop air-conditioning units that throw shade at certain sun angles.

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How do I check for shade before I get quotes?

Several free or low-cost tools let you check before an installer even turns up:

1. SunSurveyor (mobile app). Uses your phone's camera with an AR sun-path overlay: point it at the roof and see where the sun sits at 9am, noon, and 3pm across the year, and which obstructions cross the panel area. iOS and Android.
2. PVWatts (NREL's online tool). Enter the address, system size, and orientation, and it estimates annual generation, but it models clear-sky output only. Treat it as a no-shade baseline to compare real numbers against, not a shading tool in its own right.
3. The shadow test. On a clear day in June, watch a north-facing roof at 9am, noon, and 3pm and note where shadows fall. Winter shade is the worst case: clear then means clear year-round.
4. Google Earth Pro's sun slider. Free, and its sun-position slider lets you drag between December and June to see how shadow direction shifts.

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What should I ask my installer about shading?

1. "Have you assessed shading from [specific tree or structure]?" An installer who's actually walked the site will flag obstructions in the proposal. If a visible peppermint isn't mentioned, ask directly.
2. "What performance loss are you assuming from shading?" If the model assumes no shading loss and there's a tree on the block, the quoted savings are optimistic.
3. "Given the shading here, do we need power optimisers or microinverters?" Where there's partial shade, ask whether the extra cost of shade-mitigation hardware is worth the performance gain on this specific roof.
4. "What happens to the system design if a panel gets shaded?" The answer reveals whether they've actually designed around the string-drag problem or just hoped it wouldn't matter.

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Can I trim or remove a tree that's shading my panels?

Sometimes, but check with the council before assuming anything can come down.

Many Perth councils protect trees above a certain size on private land, and removing one without approval can mean a fine on top of the arborist's bill. The size threshold and the protected species vary by council and change over time, so the only reliable answer is the current rule at the specific council, not a number from a blog post.

Trimming instead of removing. Cutting back the north-facing canopy, the side actually casting shade on the roof, is often possible without a full removal application, though some councils still require a permit for significant trimming.

Native species. Marri and jarrah in particular can carry extra protections in some local planning schemes, on top of the general tree rules.

Or design around it. Sometimes the better outcome isn't removing the tree at all. Split the array across an unshaded east-west roof configuration instead, with microinverters or DC optimisers holding per-panel performance on that layout. Roof orientation has more on how east-west splits stack up against a full north-facing array.

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How much does shading actually cost me?

It depends heavily on the specific roof, so treat the maths below as an illustration, not a number to plug straight into a quote.

Say a 10kW system in Perth would generate somewhere around 15,000 kWh a year with no shade. If shading cut that by a fifth, that's roughly 3,000 kWh of lost generation a year, worth just under $1,000 a year at Synergy's A1 rate. That's the kind of number a proper shade assessment, not a guess, should be putting in front of you before you sign.

On the mitigation side, power optimisers are generally the cheaper of the two options and, unlike microinverters, can be fitted only to the panels actually sitting in shade rather than across the whole array. Ask an installer to price both ways for the specific roof: a full optimiser or microinverter fit-out, and a selective one on just the shaded panels.

In a high-shade scenario, some form of per-panel mitigation almost always earns its keep. In a low-shade scenario, it may not be worth the extra cost at all, which is exactly why the shade assessment has to come before the technology choice, not after.

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Shade from trees and roof structures is one of the most consequential variables in Perth solar performance, and installer quotes don't always spell it out. Check the roof in winter (the worst case), ask an installer how they've accounted for every obstruction on the block, and weigh shade-mitigation technology (power optimisers or microinverters) before signing. For the fundamentals beyond shade, the complete Perth solar guide is a good next stop.