How to choose the right solar system size in Perth

One of the most common questions when getting solar quotes is: "What size system do I need?" Installers typically jump straight to recommending a 6.6kW system — which is the Western Power limit for single-phase connections and happens to be what they can most easily sell you.

But the right system size depends on your consumption, how you use power, and whether you're planning to add a battery or EV charging. Here's how to work through it yourself.

Step 1 — Find your annual electricity consumption

Your Synergy bill shows your consumption for each billing period, usually in kWh. Add up the last four quarterly bills (or 12 monthly bills) to get your annual consumption.

Typical Perth household annual consumption:

| Household type | Annual kWh | |---|---| | 1–2 person apartment | 3,000–5,000 kWh | | Small home, no AC | 4,500–6,500 kWh | | Family home with reverse-cycle AC | 7,000–12,000 kWh | | Large home, ducted AC | 12,000–20,000+ kWh |

Your number from the bill is more useful than any benchmark — use your actual figure.

Step 2 — Estimate the daily consumption

Divide your annual kWh by 365 to get your daily average.

A family home consuming 9,500 kWh/year uses approximately 26 kWh/day on average. This is a rough average — winter days are higher, summer days vary depending on AC use.

Step 3 — Apply a self-consumption ratio

Not all solar generation is used in the home. Some is exported to the grid (at 2c/kWh under DEBS). Self-consumption ratio depends on your load profile:

| Situation | Self-consumption estimate | |---|---| | Home during the day (retirees, WFH, shift workers) | 60–80% | | Typical household (out 9am–5pm weekdays) | 30–50% | | EV charging from solar | +10–20% depending on charge rate | | Battery installed | 80–95% (battery absorbs the rest) |

A household with 30–40% self-consumption means 60–70% of solar generation is exported at 2c/kWh — low financial value. The solar that saves the most money is the solar you use yourself (33c savings vs 2c export).

Step 4 — Calculate the system size

Perth's peak sun hours (PSH) average 5.0 hours/day across the year (more in summer, less in winter). A 1kW solar system in Perth generates approximately 4.5–5.0 kWh/day annually.

Formula:

System size (kW) = Daily kWh target ÷ PSH

Example:

If a household wants to cover 15 kWh/day of their 26 kWh/day consumption from solar (leaving the rest to overnight/grid import):

15 kWh ÷ 5.0 PSH = 3.0 kW of solar required

But this ignores that 30–40% of generation is exported. If you want to self-consume 15 kWh, you need to generate:

15 kWh ÷ 0.40 self-consumption = 37.5 kWh generated → 37.5 ÷ 5.0 = 7.5 kW system

This is why installers often recommend 6.6kW for an average family home — it's the point at which you generate enough during the day to cover a decent portion of a typical load, with some export.

Step 5 — Check the connection limits

Single-phase connections (most Perth homes):

• Maximum inverter size: 5kW (Western Power limit for single-phase)
• Maximum panel capacity: 6.6kW (133% oversizing ratio allowed)
• This is the standard 5kW inverter / 6.6kW panel configuration you'll see quoted most often

Three-phase connections:

• Maximum inverter: 15kW (5kW per phase)
• Panels: up to ~19.9kW (133% of 15kW)
• If your switchboard has three-phase (4 main breakers rather than 2), you can install a larger system

Most Perth homes are single-phase. If you're consuming 20+ kWh/day or adding an EV, check whether your property is (or could be) three-phase connected.

Step 6 — Factor in future loads

If any of these are on your horizon within 3–5 years, size up:

• Electric vehicle: An EV charging overnight at 7.4kW draws 30–50 kWh over 4–6 hours. You can't charge this from solar directly (solar is daytime, EV is typically charged overnight), but a larger solar system with battery can store daytime solar for EV overnight charging.
• Battery: A larger system with a battery allows the battery to absorb export that would otherwise be curtailed or earn only 2c/kWh. The 6.6kW + battery combination is very common in Perth.
• Heat pump hot water: Shifting to a heat pump from gas adds 1–2 kWh/day of daytime electricity consumption — well within solar generation hours if timed correctly.
• Induction cooking: Typically lower consumption than expected — induction is very efficient. Impact: ~0.5–1 kWh/day extra, easily covered.

System size vs bill saving

| System | Annual generation | Annual saving (est.) | |---|---|---| | 3kW | 5,400 kWh | $700–$1,100 | | 5kW | 9,000 kWh | $1,100–$1,800 | | 6.6kW | 12,000 kWh | $1,400–$2,200 | | 10kW | 18,000 kWh | $1,800–$2,800 |

Savings estimates assume 40% self-consumption of generation (saving 33.26c/kWh) and 60% exported (earning 2c/kWh under DEBS). Actual savings depend on your self-consumption ratio and tariff.

The practical answer for most Perth households

For a family home consuming 8,000–12,000 kWh/year:

• Without battery now, planning to add one: 6.6kW (maximises generation within single-phase limits; battery absorbs export later)
• With battery from the start: 6.6kW + battery (captures the export that would otherwise earn 2c)
• On Midday Saver with medium self-consumption: 6.6kW (peak rate avoidance value amplifies self-consumption savings)
• EV on the way: 6.6kW or move to three-phase if eligible

For a smaller household (under 6,000 kWh/year), a 3–4.5kW system may be more appropriate — a 6.6kW system would export a very high proportion at 2c/kWh, reducing the financial return per dollar invested.

Generation estimates based on Perth PSH of 5.0 hours/day averaged annually. Self-consumption ratios are indicative; your actual ratio depends on your household's load profile. Savings estimates use A1 tariff of 33.26c/kWh and DEBS export rate of 2c/kWh effective 1 July 2026.