Perth solar panel installation: what size system do you need?
A practical guide to sizing your solar system in Perth. Work out a size from your household's electricity usage, roof space and plans, and judge it over the system's life, not the first year.
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 is a starting point. 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 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 | Starting size | Daily 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 move 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 value over the life of the system, and that is a different target. The next step explains why.
Why is self-consumed solar worth more than exported solar?
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.
That is why the right size depends so much on whether you have a battery, and when you use power. For more on shaping your day around the higher rate, see our guide to self-consumption versus exporting in Perth.
Solar only: the entry point
Without a battery, the useful target is your daytime use 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
- Solar-only system: up to 6.6 kW of panels on a 5 kW inverter, the entry point. A 5 kW inverter keeps DEBS export payments.
6.6 kW is an entry point, not a ceiling. Judge the size over the system's life rather than on how quickly it pays back: loads like an EV, a heat pump or a battery tend to arrive later, and they grow into a bigger array.
How a battery changes the size
Add a battery and the maths changes. Excess solar now charges the battery for the evening instead of being exported at the off-peak rate. That makes a bigger array worth it, and for evening-heavy homes a bigger array plus a battery often wins.
- No battery: up to 6.6 kW of panels on a 5 kW inverter
- With a battery: a bigger array. On a hybrid inverter with a battery attached, SAA Technical Requirements v1.9 §4.2 allows an array above the usual limit of about 133% of the inverter rating, up to the inverter maker's own maximum, which varies by model. A 5 kW hybrid keeps DEBS.
- With a battery and an EV: larger again, within the same inverter limits
Size the battery to your evening and overnight use. Undersizing is the costly mistake, because adding capacity later usually means a second installation. Our battery sizing guide for Perth pairs the array 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 and the usage are 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 central inverter is cheaper than per-panel micro-inverters, and a hybrid costs a little more than a plain string inverter.
- 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 should I judge the return?
Look at the value over the system's life, not just how fast it pays back:
- Net cost: the installed price after the STC deduction.
- Self-consumed solar: each kWh you use saves the grid rate, and grid prices tend to rise over time.
- Exports: paid at the DEBS rates above, on a 5 kW inverter.
- Panel life: panels keep generating for 20 years and more, sliding slowly down to roughly 80% of their original output by year 25.
A smaller system can look quicker to pay back while leaving you buying evening power for decades. Run your own numbers through the BillWise calculator and compare sizes, with and without a battery. For the cost-of-waiting angle, see the best time to buy solar in Perth.
What sizing mistakes do Perth households make?
- Sizing on the first year's payback. The size that pays back quickest is not the size worth most over its life. Judge it over the system's life, and if your evenings are heavy, price a bigger array with a battery.
- 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 hybrid inverter now lets the battery plug in later, rather than going in as an AC-coupled addition, which in WA counts as an alteration with its own gateway and compliance requirements.
- Undersizing the battery. Size it to your evening and overnight use. Adding capacity later usually means a second installation.
- 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.
The best time to go solar in Perth was five years ago. The second best time is now.
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