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

The short answer: divide your daily kWh usage by 4.25 (Perth's net yield per kW — 5 peak sun hours × 85% system efficiency), and round up to the nearest standard size. A household using 20 kWh/day needs about 5 kW. Using 25 kWh? Go 6.6 kW. But "standard" sizing misses a few things that matter — future EVs, pool pumps, batteries, and how your roof faces all change the calculation.

Quick sizing guide

As a rough rule of thumb for Perth:

| 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 |

Perth's strong solar conditions mean systems generate roughly 4.25 kWh per kW of panels per day (annual average) — 5 peak sun hours × 85% system efficiency for inverter losses, wiring, and soiling.

Step 1: Understand your usage

Check your Synergy bill for your daily average consumption. It's usually printed on the front page.

Typical Perth household usage:

2-person home: 12–18 kWh/day
Family of 4: 18–25 kWh/day
Large family: 25–35 kWh/day
With pool + AC: Add 5–10 kWh/day
With EV: Add 6–8 kWh/day

Remember that your goal isn't necessarily to cover 100% of your usage — it's to get the best financial return on your investment.

Step 2: Consider self-consumption

Here's the critical insight: solar electricity you use yourself is worth far more than solar you export.

Self-consumed solar saves you ~30+ c/kWh (avoiding grid purchase)
Exported solar earns you ~2–10 c/kWh (depending on DEBS/REBS and time)

This means a system that covers your daytime usage with minimal excess export has the fastest payback. Over-sizing leads to more exports at lower value.

The sweet spot

For most Perth households, the right system size covers your daytime baseload plus a buffer for cloudy days and winter. For a typical family:

Daytime baseload: 3–5 kWh (10am–3pm, weekday)
Plus flexible loads shifted to daytime: 3–5 kWh
Total useful self-consumption: 6–10 kWh during solar hours
Optimal system: 5–6.6 kW (generates 21–28 kWh, self-consume what you can)

If you're adding a battery

A battery changes the equation. With one, excess solar gets stored for evening use instead of exported cheaply. That makes larger systems more valuable:

Without battery: 5–6.6 kW is usually optimal
With 10 kWh battery: 6.6–10 kW makes sense
With battery + EV: 10–13 kW is ideal

Step 3: Check your roof

Roof orientation

In Perth (Southern Hemisphere), the ideal orientation is:

| 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 are actually great for Perth because they generate more power in the afternoon, when electricity demand (and time-of-use rates) are highest.

Roof space required

Each kilowatt of panels needs roughly 5–6 square metres of roof space.

| 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 |

Shading

Shading from trees, chimneys, or neighbouring buildings can sharply reduce output. Even partial shade on one panel can drag down a whole string. Your installer should run a shading analysis.

Solutions for partial shading:

Micro-inverters (each panel operates independently)
DC optimisers (same benefit, different technology)
Panel placement avoiding shaded areas

Step 4: Understand system costs

Average installed costs in Perth (2026)

| 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 |

These prices include STCs (government rebate) already deducted. Larger systems have a lower cost per kW, so they're more cost-effective if you have the roof space.

What affects price?

Panel brand: Budget (Trina, Jinko) vs premium (Aiko, REC) — SunPower/Maxeon has withdrawn from the Australian market
Inverter: String inverter vs micro-inverters
Roof complexity: Tile vs tin, single vs multi-storey, split arrays
Electrical work: Switchboard upgrade, cable runs
Installer quality: SAA-accredited installers may charge more but tend to do better workmanship

Step 5: Calculate your payback

Simple payback calculator

System: 6.6 kW for $6,500 Daily generation: ~28 kWh (6.6 kW × 4.25 kWh/kW/day) Self-consumption: ~10 kWh at 33.26 c/kWh saved = $3.33/day (Synergy A1 rate from 1 July 2026) Export: ~18 kWh, almost all before 3pm at the DEBS off-peak rate of 2 c/kWh = ~$0.36/day (DEBS pays 10 c/kWh only for exports in the 3–9pm peak window) Total daily benefit: $3.69 Annual benefit: $1,347 Simple payback: 6,500 / 1,347 = 4.8 years

After payback, the system generates free electricity for 20+ years. The return is well ahead of most everyday financial investments.

With battery addition

Add a 10 kWh battery ($10,000): Additional benefit: ~$870/year (shifting solar to evening use) Battery payback: 10,000 / 870 = 11.5 years Combined system payback: 16,500 / 2,217 = 7.4 years

Common mistakes to avoid

Over-sizing without a battery — An oversized system that exports most of its generation at low DEBS rates has a poor return. Size for your needs.
Cheapest quote wins — The cheapest system may use low-quality components. A $500 saving today could cost thousands in lost generation over 25 years.
Ignoring shading — Get a proper shading analysis. A system installed on a partially shaded roof will never perform as quoted.
Not considering future needs — Planning an EV or battery in 2–3 years? Consider a larger inverter now (even with fewer panels initially) to avoid costly upgrades later.
Skipping the switchboard check — Older homes may need a switchboard upgrade for solar. Budget $500–$1,500 if your board is pre-2000.

Get your personalised recommendation

Your optimal system size depends on your specific usage, roof, and plans. Use BillWise to get a data-driven recommendation:

Upload your bill — We'll analyse your actual consumption
Run a solar scenario — Model different system sizes and see projected savings
Get quotes from local installers — Compare prices from verified Perth installers

Start your solar assessment →

The best time to go solar in Perth was 5 years ago. The second best time is now.

Ready to act? Calculate your savings → — enter your postcode and bill amount to see what the right system size saves you.