Perth Solar Panel Installation: What Size System Do You Need?

The short answer: divide your daily kWh usage by 4.5 (Perth's average production per kW), 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 important factors — 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 | ~22 kWh/day | | 15–25 kWh | 6.6 kW | ~29 kWh/day | | 25–35 kWh | 8–10 kW | ~35–44 kWh/day | | 35+ kWh | 10–13 kW | ~44–57 kWh/day |

Perth's excellent solar conditions mean systems generate approximately 4.4 kWh per kW of panels per day (annual average).

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 maximise the 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 ~2.5–10 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 optimal 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 22–29 kWh, self-consume what you can)

If You're Adding a Battery

A battery changes the equation. With a battery, excess solar gets stored for evening use instead of exported cheaply. This 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 approximately 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 dramatically reduce output. Even partial shading on one panel can affect the entire string. Your installer should conduct 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, making them more cost-effective if you have the roof space.

What Affects Price?

Panel brand: Budget (Trina, Jinko) vs premium (SunPower, REC)
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: CEC-accredited installers may charge more but provide better workmanship

Step 5: Calculate Your Payback

Simple Payback Calculator

System: 6.6 kW for $6,500 Daily generation: ~29 kWh Self-consumption: ~10 kWh at 30.8 c/kWh saved = $3.08/day Export: ~19 kWh at 7 c/kWh = $1.33/day Total daily benefit: $4.41 Annual benefit: $1,610 Simple payback: 6,500 / 1,610 = 4.0 years

After payback, the system generates free electricity for 20+ years. The return on investment is significantly better than most 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 / 2,480 = 6.7 years

Common Mistakes to Avoid

Over-sizing without a battery — A massive 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.