Step-by-Step Guide: How to Size a Rooftop Solar + Battery System for Your Home in Perth

With electricity prices continuing to rise across Western Australia, many Perth homeowners are looking for ways to cut their energy bills and gain independence from the grid. A solar + battery system is one of the best long-term investments you can make — but to truly maximize your savings, you need to choose the right system size.

Installing a system that’s too small won’t cover your energy needs, while going too big can lead to unnecessary costs. The good news? With Perth’s abundant sunshine and the latest Federal Government Battery Rebate, now is the perfect time to get it right.

In this step-by-step guide, we’ll show you exactly how to size your rooftop solar and battery system to suit your household, future-proof your energy needs, and take full advantage of government incentives.

Step 1: Understand Your Energy Usage

The first step is knowing how much electricity your home uses each day. This determines how many solar panels and how large a battery you’ll need.

How to check your usage:

• Look at your Synergy electricity bills for average daily usage in kWh (kilowatt-hours).

• If you have a smart meter, download your usage data to see patterns — daytime vs. nighttime usage.

• Estimate based on typical household sizes:

If your family uses 22 kWh/day, that’s roughly 8,030 kWh per year.

Knowing this number will guide every decision from here on.

Step 2: Factor in Perth’s Solar Conditions

Perth is one of the sunniest cities in Australia, making it ideal for solar energy production.

• Average peak sun hours: 5.5 – 6 hours per day

• This means a standard 6.6kW system can generate approximately 26 – 28 kWh/day.

Because Perth gets so much sunshine, you can achieve excellent energy output with a moderate-sized system, reducing your payback period significantly.

Step 3: Sizing Your Solar System

The formula to estimate the right system size is simple:

Solar Size (kW) = Daily Energy Use (kWh) ÷ Peak Sun Hours

Example Calculation:
If your home uses 20 kWh/day and Perth has 5.5 peak sun hours:
20 ÷ 5.5 = 3.6 kW (round up to 4 – 5 kW to allow for future growth).

Why most Perth households choose 6.6kW:

• Matches the standard single-phase inverter limit.

• Eligible for maximum federal rebates under the STC program.

• Produces enough energy for a medium family, plus some excess for battery charging.

Pro Tip:
Always size a little bigger if you plan to get an EV or expect higher usage in the future.

Step 4: Sizing Your Battery

A battery stores excess solar energy for nighttime use or during grid outages.
The formula for battery sizing is based on evening and overnight consumption:

Battery Size (kWh) = Evening Usage (kWh)

Typical Battery Sizes:

Example:
If your household uses 9 kWh after sunset, a 10 kWh battery will be ideal.

Step 5: Take Advantage of the New Federal Solar Battery Rebate

The Australian Federal Government’s new battery rebate program launched in 2025, making battery storage much more affordable for WA homeowners.

Here’s what you need to know:

• Rebate Amount: Up to $330 per kWh of usable battery capacity.

• Maximum Support: Up to 50 kWh per household, which equals $17,000 in savings.

• Eligible Systems: Must be installed by an accredited installer and connected to solar panels.

• Goal: Encourage battery adoption to reduce grid demand during peak times.

Example Savings:

• 10 kWh battery → $3,300 rebate

• 15 kWh battery → $4,950 rebate

• 20 kWh battery → $6,600 rebate

Combined with Perth’s STC solar rebates, you can slash the upfront cost of a full solar + battery system by nearly 50%.

Tip:
Demand for this rebate is high. Booking early ensures you don’t miss out.

Step 6: Plan for Future Energy Needs

When sizing your system, think ahead:

• Electric Vehicles: Add 8–12 kWh/day for charging.

• New appliances: Pool pumps, ducted air conditioning, induction cooktops.

• Home renovations: Granny flats or home offices.

Adding 20–30% extra capacity now is far more cost-effective than upgrading later.

Step 7: Get a Professional Solar Assessment

Before committing, have a CEC-accredited installer conduct a site visit to evaluate:

• Roof angle and orientation for maximum sun exposure.

• Potential shading from trees or nearby buildings.

• Space for battery installation (indoor vs. outdoor).

• Any necessary electrical upgrades.

A tailored system design ensures you get the highest return on investment and long-term performance.

Sample Sizing Scenarios for Perth Homes

Why Now Is the Best Time to Go Solar in Perth

• Perth has some of the best solar conditions in Australia.

• Federal battery rebate + STC rebates can cut your upfront cost in half.

• Electricity rates are forecast to increase further over the next few years.

• Adding a battery now gives you protection against blackouts and rising grid charges.

By sizing your system correctly today, you’ll lock in energy independence and decades of savings.

Work with a Trusted Local Solar Expert

When it comes to installing a solar + battery system, choosing the right installer is just as important as selecting the right system.

Empower Solar Australia is a CEC-approved leading solar installer in Perth, known for high-quality solar solutions, exceptional customer service, and end-to-end support — from system design to installation and ongoing maintenance.

With Empower Solar Australia, you’ll get:

• Expert system design tailored to your household needs.

• Access to premium panels and batteries, including AlphaESS and Tesla Powerwall.

• Guidance on maximizing rebates and incentives.

• A seamless installation experience by fully accredited professionals.