What Size Solar Battery Do I Need? A Step-by-Step Worksheet

Most battery quotes start with a size and work backwards. "A 13.5 kWh battery suits a home like yours." Maybe it does. But nobody can tell you that without looking at when your house actually uses power, and you can work most of it out yourself with a recent bill or your retailer's app and ten minutes.

The size of solar battery you need is roughly your average power use between about 4pm and 7am, less any loads you don't want the battery to carry, plus a reserve if you want blackout backup. Then check your panels make enough surplus to fill it, especially in winter, and round to the nearest module size. For most Sunshine Coast homes that lands between 8 and 16 kWh, which is why 10 kWh and 15 kWh are the sizes we quote most often.

Below is the worksheet we use, step by step, with a worked table at the end.

Updated 2 November 2026 with the current battery rebate rate and the 1 January 2027 step-down.

How do I work out what size battery I need?

Five steps. Grab your bill, or log in to your retailer's app or website, and if you already have solar, open your inverter app too.

1. Find your average 4pm to 7am usage.
2. Take out the loads you don't want the battery to cover.
3. Add a reserve if you want backup in a blackout.
4. Check your solar array can refill it.
5. Round to a real battery size.

Step 1: Find your 4pm to 7am usage

This is the number that matters most, and it's not your daily total.

Your daily total includes everything you use while the sun's up, and much of that is covered by your panels directly. A battery's job is the part the panels can't cover: from late afternoon, when solar output fades, through to early morning.

How to find it:

• Interval data. If you have a smart meter, your retailer's app or online account can usually show your use in half-hour or hourly blocks. Add up roughly 4pm to 7am for a typical day. Better still, look across a few weeks, including a hot summer stretch and a cold winter one.
• Inverter app. If you already have solar with consumption monitoring, it'll show your evening and overnight use directly.
• No interval data? Your bill's daily average gives you an upper limit, but on its own it will oversize the battery. Ask your installer to pull the interval data with your permission; any decent one will.

Use a typical night, not your worst night. A battery sized for the hottest night of February sits partly empty for most of the year.

Step 2: Take out what you don't want the battery to cover

Not every kilowatt-hour after 4pm needs to come out of the battery. Common ones to take out:

• Hot water or a pool pump on a controlled-load tariff. These are metered separately (you'll see a separate line on your bill) and already run on cheaper power. Most people leave them out of the battery sum. Check with your installer how yours is wired.
• Loads you can move into the day. A pool pump, dishwasher or washing machine on a timer in the middle of the day runs on solar directly and never touches the battery.
• EV charging, if the car can be charged in the solar window instead. If it can only charge overnight, it stays in, and it'll probably push you into the bigger sizes.

What's left is the energy the battery has to supply on a normal night.

Step 3: Allow a reserve for blackouts

If you want the battery to keep the lights, fridge and internet going during an outage, you'll usually set it to hold some charge back rather than run flat every night. Sunshine Coast storm season is the usual reason people want this.

That reserve isn't available for everyday use, so it has to be added on top. The sum is:

Usable capacity needed = nightly energy ÷ (1 − reserve)

For example, with 9 kWh to cover each night and a 10% reserve: 9 ÷ 0.9 = 10 kWh. With a 20% reserve on 18 kWh: 18 ÷ 0.8 = 22.5 kWh.

If you don't care about backup, skip this step. Also worth knowing: what runs in a blackout depends on how the backup circuits are wired and the inverter's power rating (kW), not just the battery's size (kWh). Our guide on how long a 10 kWh battery lasts explains the difference with worked runtimes.

Step 4: Check your panels can refill it

This is the step most online battery calculators skip, and it's the one that catches people out. A battery doesn't make energy. It stores the surplus your panels produce after the house has used what it needs. If there isn't enough surplus, the extra battery sits empty.

If you already have solar: look at how much you export to the grid each day. That export is the most a battery could catch. Look at winter months (June and July) as well as summer, because the surplus shrinks when the days are short.

• Exporting more than your battery size on most days, including winter? The battery will fill. Good.
• Exporting less than your battery size in winter? The battery will often sit partly empty for those months. You can live with that, go a size smaller, or add panels.

If you're getting solar and a battery together: the array gets designed to suit the battery. As a rough guide from our installs, a 6.6 kW array is about the minimum we'd pair with a 10 kWh battery on a typical Sunshine Coast home, and bigger batteries need bigger arrays. If you're weighing up where to spend, our guide to more solar panels or a bigger battery works through it.

Step 5: Round to a real battery size

Batteries don't come in any size you like. Most systems we install stack in modules of roughly 4 to 5 kWh, so your number gets rounded to the nearest combination. A few examples from brands we install:

• Deye high-voltage modules are 5.12 kWh each, so 10.24, 15.36 or 20.48 kWh.
• Sigenergy stacks modules into systems from about 8 kWh up to about 48 kWh.
• Tesla Powerwall is a fixed 13.5 kWh per unit.

Two rules for rounding:

• Use the usable capacity, not the number in the product name. A battery called "10 kWh" may have a little less usable. The rebate is also calculated on usable capacity.
• If you're between two sizes, lean towards the smaller one, unless you know a bigger load is coming (an EV, a pool, a granny flat). Oversizing is the most common mistake we see on quotes, and the last few kilowatt-hours of an oversized battery only get used on your heaviest nights.

There's one catch on "add more later", though. Under the Cheaper Home Batteries Program it's one battery rebate per property, and the Clean Energy Regulator is clear that you can't claim again if you upgrade to a larger system later. Modules added to a battery that's already claimed get no rebate. So if an EV is genuinely arriving next year, it can make sense to include that capacity now. If it's a maybe in five years, don't pay for it today.

Worked example: three households

These are made-up example numbers to show the method. They aren't measurements from real homes; your own figures come from your bill or app.

How to read it:

• Household A needs about 6 kWh. A single module of around 5 kWh covers most nights with a little grid top-up on heavy ones; a battery in the 5 to 6.5 kWh range covers the lot. Going to 10 kWh would mostly buy empty capacity.
• Household B is the classic 10 kWh home. The panels fill it even in winter, and the reserve covers the essentials in a storm outage.
• Household C wants 22.5 kWh on paper, but the array only exports about 14 kWh on a winter day. A 15 kWh battery fits what the panels can actually fill, stays mostly inside the full-rate rebate tier, and leaves the option of a bigger array later. Going straight to 20 kWh-plus only makes sense with more panels to fill it.

How does the battery rebate affect what size I need?

It shouldn't decide the size, but the 14 kWh tier is worth knowing about. The federal rebate is calculated per usable kWh, in three tiers:

Those figures use 6.8 STCs per kWh now and 5.7 from 1 January 2027, at an STC price of about $37 (6.8 × $37 ≈ $252; 5.7 × $37 ≈ $211). The STC price floats, so treat them as approximate.

The sizing point: every kilowatt-hour above 14 earns 60% of the rebate, and above 28 only 15%. If your worksheet says 15 or 16 kWh, fine, the extra kWh still earns a decent share. If someone is pushing you from 15 to 25 kWh "because of the rebate", the rebate is doing a lot less work than they're implying. Our post on the January 2027 battery rebate changes shows the full arithmetic by battery size, and the solar battery cost guide shows what each size costs installed.

The battery also has to be at least 5 kWh to qualify, so going below that for a very small home means no rebate at all.

Are online solar battery calculators accurate?

They're a reasonable starting point, but only as good as what you put in. Most take your daily or quarterly use, apply an average evening share and give you a size, without asking what your panels export in winter.

The worksheet above is the same calculation done with your own numbers, which is why it's more reliable. The Clean Energy Regulator's solar battery STC calculator is useful too, but for a different job: it estimates the rebate certificates, not the size you need.

Quick guide: what size battery suits which home

If you just want a ballpark before you dig out a bill:

Off-grid is a different calculation altogether. Without the grid as a backstop you're sizing for several days of cloud, not one night, and the numbers go up a lot. Our guide to off-grid battery sizing covers that method.

What we look at when we size a battery

When we quote a battery, we do the same five steps with your interval data, plus the things a worksheet can't see: your switchboard, your existing inverter, where the battery can legally go, and what's coming in the next few years. Sometimes the honest answer is a smaller battery than you expected, or more panels first and a battery later.

If you'd like us to run the numbers for your house, our solar battery installation service starts with your usage data, not a stock size.

Frequently asked questions

What size solar battery do I need?

Roughly your average use between 4pm and 7am, less loads the battery won't cover (like controlled-load hot water), plus a reserve if you want blackout backup. Then check your panels export enough to fill it in winter and round to the nearest module size. Most Sunshine Coast homes land between 8 and 16 kWh.

How much battery storage do I need for my house?

For a typical family home, about 10 kWh. Units and low-use couples often need under 6.5 kWh, larger homes with a pool or ducted air about 15 kWh, and 20 kWh or more only makes sense with overnight EV charging or a big array to refill it.

Is a 10 kWh battery enough?

For many family homes, yes. If your 4pm to 7am use is around 8 to 10 kWh and your panels export more than that on a winter day, a 10 kWh battery will cover most nights. Heavy overnight air conditioning, a pool pump running at night or EV charging can push you past it.

Can I add more battery capacity later?

Physically, often yes, because most batteries are modular. But it's one battery rebate per property, so modules added later to a battery that already claimed the rebate get no rebate. If a bigger load is definitely coming soon, it can be worth sizing for it now.

How do I calculate my battery size from my electricity bill?

Use your retailer's interval data to add up your use from about 4pm to 7am on a typical day. If you only have the bill's daily average, that's an upper limit, not your answer, because part of it happens while your panels are producing. Your installer can pull the interval data with your permission.

What size battery do I need for a blackout?

Size it for your normal nights first, then add a reserve that's held back for outages, for example 10 to 20% of capacity. What actually runs in a blackout depends on which circuits are wired for backup and the inverter's power rating, so tell your installer what you want kept on.