What Size Solar Battery Do I Need?

The word “size” in solar battery doesn’t refer to physical dimensions, but rather how much energy the battery can store. The unit of size for a solar battery is kilowatt-hours (kWh).

kWh is a unit of energy, not power. If you have a solar battery with 10 kWh usable capacity, it can store enough electricity to power ten 1,000-watt appliances for one hour or one 1,000-watt appliance for ten hours. Of course, in a real home scenario, power usage is spread across many smaller appliances running at different times. These appliances could be your fridge, lights, internet modem, etc. 

Why understand all these and not leave it all up to your solar provider?

It is because the right battery size depends on your habits, lifestyle, and energy goals. Solar providers can give recommendations, but when you understand how battery sizing works, you’re in a better position to ask the right questions, avoid upselling, and ensure the solution truly fits your needs, not just your installer’s default option.

Your reason for installing a solar battery will directly shape what size is right for your home. The best size depends on what you want the battery to do. Here’s how different goals can affect the battery size you need:

• Energy independence – Requires a medium to large battery, depending on how much solar you generate and how much you use at night.

• Backup power – The battery size depends on what appliances you want to keep running and for how long. A small battery might be enough for lights and Wi-Fi, while a whole-home backup needs a much larger system.

• Bill savings – You’ll want a battery that matches your excess solar production and evening consumption, usually in the small to medium range.

• Future planning (e.g. EV, home office) –  It needs a larger battery or a system that’s easy to expand.

Clarifying your main goal early on makes it easier to discuss your battery needs with your solar installer.

This section will walk you through a simple step-by-step guide, using an example. You’ll learn how to factor in your energy usage, nighttime needs, battery efficiency, backup power preferences, and plans for the future.

Before you begin, just remember: your solar battery is only useful if your solar system produces enough excess electricity to fill it. So, as a rule of thumb, your system’s excess solar generation is greater than or equal to your estimated battery size.

Every home is different, and so is its energy usage. We've made a quick reference guide below to help you get a feel for the battery size that might suit your household. These are only estimates. Your actual needs may vary based on habits, appliances, and solar generation.

Your battery type determines how much stored energy you can use, how long the battery lasts, and how safely and efficiently it runs. By understanding these differences, homeowners like you can make better decisions about how much capacity you need and avoid overpaying or undersizing your system. Here are the most common chemistries found in Australian home batteries today:

When sizing a solar battery, always look beyond the label. A 10 kWh battery doesn’t always mean you’ll get 10 kWh of usable energy, and that’s due to system efficiency losses.

Solar Panel Orientation

The direction your solar panels face affects how much energy they generate. North-facing panels produce the most energy, while east or west-facing panels produce less.

Since a battery typically stores the excess energy your panels generate, it’s better to size your battery based on your excess solar generation and energy needs. Oversizing your battery when your solar panels produce less energy often leads to underutilised storage and unnecessary cost. Instead, focus first on optimising your system’s energy production or adjusting consumption habits for better efficiency.

Choosing the right size for your solar battery is also based on how much flexibility and future-proofing you want. Here’s a quick look at the trade-offs between undersizing and oversizing your system:

A well-sized or slightly expandable battery gives you the best balance between cost and capability. If you're unsure about sizing, consider a battery system that allows modular expansion. Starting with a smaller battery and adding more later can be a smart way to manage cost while future-proofing your system.

Once you’ve done your homework and have a rough idea of the battery size you might need, the next step is to have an informed conversation with your solar installer. Here are the key questions you should ask to ensure the system recommended is the right fit, not too small, not unnecessarily large, and certainly not more expensive than it needs to be.

Ask for a scenario based on your specific energy habits. This helps you understand what to realistically expect once the battery is installed.

Life changes. Working from home, getting an EV, or expanding the household affects electricity use. Ask if the battery setup can grow with your future needs.

A trustworthy installer should walk you through the reasoning behind the battery size they recommend, including how they assessed your daily usage, solar generation, night-time needs, and backup goals.

Not all 10 kWh batteries are created equal. Depending on the Depth of Discharge, you may only get to use 8 or 9 kWh. Ask about usable capacity, not just the total size.

Your state may offer rebates, interest-free loans, or subsidies. Ask your installer to walk you through any available programs that can reduce your upfront cost or ongoing repayments.