Perth solar panel installation: what size system do you need?

Divide your average daily kWh by 4.25 and round up to the nearest standard system size. That 4.25 is Perth's net yield per kW of panels each day: 5 peak sun hours times 85% system efficiency. A home using 20 kWh a day needs about 5 kW. On 25 kWh a day, go to 6.6 kW. On 35 kWh or more, you are into 10 kW and up.

That rule gets you 90% of the way. The rest depends on what your household is about to become. A future EV, a pool pump, a battery, and the way your roof faces all move the number, usually upward. This guide walks through each step so the size you order still fits in five years, not just this quarter.

How do I work out the right solar size for my usage?

Start with one figure: your average daily consumption in kWh. Synergy prints it on the front page of your bill. Divide it by 4.25, round up, and you have a sensible starting size.

Perth's strong sun means panels generate roughly 4.25 kWh per kW per day across the year, which is 5 peak sun hours times 85% system efficiency. The 85% covers the unavoidable losses: the inverter, the wiring, a little dust on the glass. Here is what that looks like by usage band.

Daily usage	Recommended system	Annual generation
10–15 kWh	5 kW	~21 kWh/day
15–25 kWh	6.6 kW	~28 kWh/day
25–35 kWh	8–10 kW	~34–43 kWh/day
35+ kWh	10–13 kW	~43–55 kWh/day

These are starting points, not final answers. Read on for the things that nudge them.

What does a typical Perth household actually use?

Pull your daily average off the bill, then sanity-check it against the household types below. If yours sits well above the band you expected, something is driving it: a pool, ducted air conditioning, or an electric hot water system on the wrong tariff.

Two-person home: 12–18 kWh/day
Family of four: 18–25 kWh/day
Large family: 25–35 kWh/day
Add a pool plus air conditioning: 5–10 kWh/day on top
Add an EV charged at home: 6–8 kWh/day on top

Your goal is not to cover 100% of usage on paper. It is to get the best financial return, and that is a different target. The next step explains why.

Why is self-consumed solar worth more than exported solar?

This is the single insight that decides the right size. Solar you use yourself is worth far more than solar you sell back.

Self-consumed solar saves you the full grid rate you would otherwise pay, around 30 c/kWh or more.
Exported solar earns the DEBS feed-in rate: 2 c/kWh off-peak, and 10 c/kWh only for exports in the 3pm to 9pm peak window.

A system that covers your daytime use with little excess export pays itself off fastest. Oversize it and you simply sell more power at the low rate. For more on shaping your day around the higher rate, see our guide to self-consumption versus exporting in Perth.

Where the sweet spot sits

For most Perth households the right size covers your daytime baseload plus a buffer for cloudy days and winter. A typical family looks like this:

Daytime baseload: 3–5 kWh between 10am and 3pm on a weekday
Flexible loads shifted into daylight: another 3–5 kWh (dishwasher, washing, pool pump)
Useful self-consumption during solar hours: 6–10 kWh
System that fits: 5–6.6 kW, generating 21–28 kWh, with you using what you can

How a battery changes the size

Add a battery and the maths flips. Excess solar now charges the battery for the evening instead of being exported at 2 cents. That makes a bigger array worth it.

No battery: 5–6.6 kW is usually optimal
With a 10 kWh battery: 6.6–10 kW makes sense
With a battery and an EV: 10–13 kW

If a battery is on your list, our battery sizing guide for Perth pairs the array size with the right storage capacity.

Will my roof fit the system I need?

Three roof factors decide whether your ideal size is realistic: which way the panels face, how much room you have, and what is casting shade.

Which way should the panels face in Perth?

Perth sits in the Southern Hemisphere, so north is best. The table below is generation relative to a north-facing array.

Direction	Suitability	Generation vs north
North	Optimal	100%
North-west	Excellent	~95%
North-east	Excellent	~95%
West	Good	~85%
East	Good	~85%
South	Poor	~65%

West-facing panels punch above their rating in Perth. They generate most in the afternoon, exactly when household demand and time-of-use rates climb. Our west-facing solar guide and the broader roof orientation guide go deeper on splitting an array across two aspects.

How much roof space does each system need?

As a rough rule of thumb, each kilowatt of modern panels needs about 5 to 6 square metres of roof.

System size	Approximate roof area
5 kW	25–30 m²
6.6 kW	33–40 m²
10 kW	50–60 m²
13 kW	65–78 m²

Panel efficiency varies by product, so check the datasheet area for the exact models on your quote.

What does shading do to output?

Shading from trees, chimneys, or a neighbour's roofline can cut output sharply. Even partial shade on one panel can drag down a whole string. A proper shading analysis before you sign is worth more than any spec-sheet promise, and our shading analysis guide shows what a good one covers.

Three ways to handle a shaded roof:

Micro-inverters, so each panel works independently of the others
DC optimisers, the same benefit through different hardware
Panel placement that simply avoids the worst-shaded areas

What does a solar system cost in Perth in 2026?

Installed prices below already have the federal STC rebate deducted, so they are close to what you pay out of pocket.

System size	Price range	Cost per kW
5 kW	$4,500–$6,500	$900–$1,300
6.6 kW	$5,500–$8,000	$830–$1,210
10 kW	$8,000–$12,000	$800–$1,200
13 kW	$10,000–$15,000	$770–$1,150

Bigger systems cost less per kW, so if the roof space is there, scaling up is usually the better value.

What changes the price?

Panel brand: budget ranges like Trina and Jinko sit below premium names like Aiko and REC.
Inverter: a single string inverter is cheaper than per-panel micro-inverters.
Roof complexity: tile costs more than tin, two storeys more than one, split arrays more than a single plane.
Electrical work: a switchboard upgrade or a long cable run adds cost.
Installer quality: an SAA-accredited installer may charge a little more and tend to do better work. Checking accreditation is the first step in our solar buying checklist.

How do I calculate solar payback in Perth?

Here is a worked example for a common 6.6 kW system, using current Synergy rates.

System: 6.6 kW for $6,500
Daily generation: about 28 kWh (6.6 kW times 4.25 kWh per kW per day)
Self-consumption: about 10 kWh, saving 33.26 c/kWh on the Synergy A1 rate, which is $3.33 a day
Export: about 18 kWh, almost all before 3pm at the DEBS off-peak rate of 2 c/kWh, which is about $0.36 a day
Total daily benefit: $3.69
Annual benefit: about $1,347
Simple payback: 6,500 divided by 1,347, about 4.8 years

After payback the panels keep generating for 20 years and more, sliding slowly down to roughly 80% of their original output by year 25. The return comfortably beats most everyday investments. For the cost-of-waiting angle, see the best time to buy solar in Perth.

What if I add a battery?

Add a 10 kWh battery at around $10,000 and the picture shifts toward evening savings.

Extra annual benefit: roughly $870 from shifting stored solar into the evening (figures vary with your usage pattern and tariff)
Battery payback on its own: 10,000 divided by 870, about 11.5 years
Combined system payback: 16,500 divided by 2,217, about 7.4 years

What sizing mistakes do Perth households make?

Oversizing without a battery. A big array that exports most of its output at 2 cents has a slow return. Size for what you use, not what fits.
Letting the cheapest quote win. The lowest price often hides low-grade components. A $500 saving now can cost far more in lost generation across 25 years.
Ignoring shading. Get the analysis done. A system on a partly shaded roof never performs the way the quote promised.
Forgetting what is coming. Planning an EV or battery in the next few years? A larger inverter now, even with fewer panels to start, saves a costly upgrade later.
Skipping the switchboard check. Older homes sometimes need a switchboard upgrade before solar goes on. If your board predates 2000, budget for one; a switchboard upgrade in Perth typically runs a few hundred to around fifteen hundred dollars.

Get a recommendation built on your own numbers

The right size comes down to your usage, your roof, and your plans. BillWise turns those into a specific recommendation:

Upload your bill so we can read your actual consumption.
Run a solar scenario to model different sizes against projected savings.
Get quotes from local installers and compare verified Perth prices.

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The best time to go solar in Perth was five years ago. The second best time is now.

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