The short answer
Most Perth homes suit a 6.6kW solar system with a 5kW inverter. To size yours, divide your average daily electricity use (kWh) by 4.4, which is roughly what each kilowatt of panels produces per day in Perth. A home using 20kWh a day needs about 4.5kW to match its usage on paper, and most households go to 6.6kW to cover winter and future needs like an EV.
If you’re out during the day, a battery sized to your evening use is usually where the bigger savings come from.
How to Work Out Your Solar System Size. (Step by Step)
Find your daily electricity use
Take the total kWh from your Synergy bill and divide it by the number of days in the billing period. A bill showing 1,200kWh over 60 days means you use 20kWh a day. Use a year of bills if you can, because summer and winter usage can be very different.
Divide by Perth’s solar output
In Perth, each kilowatt of panels produces about 4.4kWh a day averaged across the year, closer to 5.5kWh in summer and under 3kWh in mid-winter. So 20kWh ÷ 4.4 = 4.5kW of panels to match your usage on an average day.
Check your roof and power supply
Each 440W panel needs about 2m² of roof. Single-phase Perth homes are usually limited to a 5kW inverter, which pairs with up to 6.6kW of panels. Three-phase homes can usually go larger, subject to Western Power approval.
Size a battery to your evenings
Work out how much you use after the sun goes down, typically 50–70% of the day’s total. A home using 20kWh a day with 12kWh after dark suits a battery of around 10–13.5kWh, provided the panels make enough spare daytime energy to fill it.
Typical Solar System Sizes for Perth Households
| Household | Daily use | Solar system | Battery |
|---|---|---|---|
| 1–2 people, out during the day | 8–12kWh | 3–5kW | 5–10kWh |
| 3–4 people, typical family home | 15–22kWh | 5–6.6kW | 10–13.5kWh |
| 4+ people with pool or ducted air con | 22–30kWh | 6.6kW (single-phase) or 8–10kW (three-phase) | 13.5kWh or more |
| Any home plus an EV | Add ~7kWh | Step up one size, or charge during the day | Size to evening use, not the car |
A guide only. Your roof, shading and switchboard decide the final size, which we confirm at a free site assessment.
What Changes the Size You Need
Roof direction and shade
North-facing roofs produce the most over a year. West-facing panels produce a little less in total but more in the late afternoon, which lines up well with when Perth homes use the most power. Shade from trees or a neighbour’s second storey can mean you need more panels for the same output.
When you use power
Solar is worth the most when you use it as it’s made, because WA feed-in credits are small compared with what you pay for grid power. If you’re home during the day, panels alone go a long way. If you’re out, a battery or timers on the pool pump and dishwasher make a bigger difference than extra panels.
What’s coming next
An EV charger, a new pool, switching from gas to electric appliances, or kids growing into teenagers all push usage up. It’s cheaper to size for the next five years now than to add a second system later.
How to Size a Solar Battery
A battery should be sized to two things: how much power you use after sunset, and how much spare solar your panels make during the day to fill it. A large battery on a small system never fully charges, especially in winter.
As a rough guide, most Perth family homes suit 10–13.5kWh of storage, which is the capacity of a single Tesla Powerwall 3. Smaller households are often well served by 5–10kWh. If your existing system is 5kW or less, it’s worth checking whether you need more panels before adding a battery.
We check your eligibility for the Federal Battery Rebate and current incentives as part of every quote.
Solar System Size Calculator
Answer four quick questions. You’ll see a suggested system size, battery size, and how your usage would be covered through a Perth year.
Tell us about your home
Results update as you go. Nothing is sent anywhere.
How do you want to start?
People in your home
Do you have any of these?
Your average daily usage
On your Synergy bill, divide the total kWh used by the number of days in the billing period.
Is someone usually home during the day?
Your power supply
Most Perth homes are single-phase. Three-phase is common in larger or newer homes, and ones with ducted air con.
How your daily usage would be covered
What this system makes through a Perth year
Get this checked at a free site assessmentA guide only. Based on typical Perth output of about 4.4 kWh per kW per day, a north-facing unshaded roof and 440W panels. Your actual size depends on your roof, shading, switchboard and Western Power approval, which we confirm on site.
How We Size Your System at the Free Site Assessment
1
Review your bills
We look at a full year of usage, not one bill, so summer and winter are both covered.
2
Measure the roof
We check direction, pitch, shading and usable space for every roof face.
3
Check the switchboard
We confirm single or three-phase supply and whether your board needs an upgrade.
4
Quote a clear size
You get one recommended solar and battery size, with the reasoning written down.
Solar System Sizing: Common Questions
Divide your average daily usage in kWh by 4.4. A home using 20kWh a day needs about 4.5kW to match its usage, and most Perth homes install 6.6kW to cover winter and future needs. Use the calculator above for an estimate based on your household.
For most Perth homes using 15–25kWh a day, yes. A 6.6kW system makes around 29kWh on an average day, from about 18kWh in mid-winter to 37kWh in summer. Homes with a pool, ducted air con and an EV may need a larger three-phase system or a battery to get the most from it.
Single-phase homes on the Western Power network are usually limited to a 5kW inverter. Installing up to a third more panel capacity than the inverter rating (6.6kW on a 5kW inverter) is allowed and keeps the inverter working near capacity for more of the day.
Divide the system size by the panel wattage. A 6.6kW system uses 15 × 440W panels, and a 5kW system uses about 11–12. Each panel needs roughly 2m² of roof space.
A little headroom is sensible, especially if an EV, pool or electric heating is likely in the next few years. Going far beyond your usage without a battery mostly adds exported power, which earns a small feed-in credit in WA.
Size it to your evening and overnight usage, which is usually 50–70% of your daily total. Most family homes suit 10–13.5kWh. Your panels also need to make enough spare daytime energy to fill it.
A typical daily commute adds around 5–8kWh. If you can charge during the day, extra panels cover it well. If the car charges overnight, a battery or a larger system with a smart EV charger helps.
Often yes, but it depends on your inverter’s spare capacity and your network limits. Matching older panels can also be tricky, so it’s usually cheaper to size correctly the first time. We can assess an existing system during a free site visit.
Yes. North-facing panels produce the most over a year. East and west-facing roofs produce a little less in total, so you may need an extra panel or two, but west-facing panels produce more in the late afternoon when many homes use the most power.
It’s a good starting point based on typical Perth output and household usage. Your final size depends on your roof, shading, switchboard and actual bills, which we check at a free site assessment.
Get a solar system that suits your needs
Not sure which solar system suits your needs? The team at Solar2020 are here to help and available to answer your questions.
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