How to Choose a Solar Battery Size: Everything You Need To Know

Figuring out what size solar battery you need shouldn’t feel like learning a new language. Between “overnight usage,” “export limits,” and “peak sun hours,” it’s easy to feel like the more you read, the less you know.

And yet—battery size is one of the most important decisions you’ll make when going solar. Too small, and it’ll run out before your evening’s even over. Too big, and you might be paying for capacity you’ll never use. Plus, with the new federal rebate only claimable once, getting it wrong now could cost you later.

At Lenergy, we’ve helped thousands of homeowners size their solar and storage systems based on real-life usage—not guesses or generic recommendations. We know how to spot when a 10kWh battery makes sense, and when a 24 or 48kWh system would actually serve you better in the long run.

In this guide, you’ll learn exactly how to choose the right battery size for your home. We’ll walk through the formulas, examples, and trade-offs—so you can decide what fits your needs, your lifestyle, and your roof.

Why Battery Size Isn’t One-Size-Fits-All

There’s no universal “right” battery size. The best fit depends on two things:

1. Whether you already have solar or not
   
2. How and when your household uses power
   

If You’re Starting Fresh: Solar + Battery

If you’re installing both panels and a battery, you can size the entire system around your needs. This means designing your solar array large enough to cover daily usage and charge the battery — especially in winter when generation drops and usage usually increases  (this is the case in the Southern Highlands as it is colder and the AC heater is generally running more frequently. Though in warmer climates such as Newcastle, their energy use is quite stable and constant throughout the year)

It also means you can plan ahead. Thinking about buying an EV? Having Kids? Upgrading air con? Taking on housemates? Starting from scratch gives you flexibility to factor those changes into the design.

If You Already Have Solar: Battery Retrofit

Adding a battery to an existing system is a bit different. Here, the biggest question is: Do you have enough solar exports to fill a battery?

If you’re only exporting 2–3kWh a day, there’s not much spare energy to store. But if you’re exporting 10kWh or more, a battery starts to make a lot more sense.

What Size Solar System Do You Need to Charge a Battery?

If you’re installing solar for the first time, it’s not just about powering your home — it’s about generating enough energy to charge your battery too. This is where a simple sizing formula can help.

Step 1: Find Your Daily Energy Usage
(usually on page 2 or 3 of your bill – see below snippet)

Your electricity bill will show your average daily usage, measured in kilowatt-hours (kWh). The above is 20kWh / day.

Let’s use another example and say it’s 24kWh/day — a common number for a family home.

Step 2: Estimate Your Local Sun Hours

Solar panels don’t generate their rated output all day long. In most parts of Australia, we use a conservative estimate of 3.6 peak sun hours (PSH) — the number of hours per day when solar panels operate at close to their rated capacity.

Step 3: Use the Sizing Formula

Solar system size (kW) = Daily usage (kWh) ÷ 3.6 (PSH)

For example:

24kWh ÷ 3.6 PSH = 6.6kW solar system

This means you’d need a 6.6kW system to roughly match your daily energy needs — assuming average weather across the year.

But remember: winter generation is lower, so it often makes sense to oversize slightly to ensure your system can still charge your battery during shorter, cloudier days.

If You Don’t Have Solar Yet: How to Estimate Your Battery Needs

If you haven’t installed solar yet, your electricity bill is your best clue — but it doesn’t tell the full story. It shows how much power you use each day, but not when you use it. That matters, because batteries only cover your night-time usage (when solar isn’t running).

So, how do you estimate battery size without solar data?

General Rule of Thumb

Most households use a good chunk of their energy at night — especially if everyone’s out during the day. Here’s how to estimate:

• Light to moderate night-time usage:
  Battery size ≈ 50% of daily usage
  
• Heavy night-time usage (away during the day):
  Battery size ≈ 75% of daily usage
  

Example 1: Bill Without Solar

From the Lenergy example:

• Average daily usage: 24kWh
  

Option 1 – Moderate night use:
24 × 0.5 = 12kWh battery

Option 2 – Higher night use:
24 × 0.75 = 18kWh battery

So if this home adds solar + battery, either size could work — but we’d also want to check their lifestyle, winter usage, and future energy needs before finalising anything.

Keep in mind: If you size your battery before installing solar, you’ll want to make sure your panel system is big enough to charge the battery as well. That’s where the 6.6kW example from the last section fits in.

If You Already Have Solar: Can Your System Fill a Battery?

If you’ve already got panels on your roof, you don’t have to guess. Your electricity bill holds the answers — specifically in the solar export and daily usage numbers.

These tell you whether you’re generating enough excess solar to charge a battery and how much storage you actually need.

Step 1: What to Look for on Your Bill

Find these two figures:

• Average daily usage (kWh) — your total electricity use
  
• Average daily export (kWh) — how much solar you’re sending back to the grid
  

If you’re exporting less than 5kWh/day, there may not be enough spare energy to charge a battery — at least not consistently. If you’re exporting 10–15kWh/day or more, you’re in good shape to store that energy instead of selling it back for cents.

Example 2: Existing Solar, Moderate Use

From the bill shown below:

Key takeaways:

• General usage = $0.29 / kWh
• Daily supply charge = $1.30
• Average daily usage = 13.75kWh
• Average daily export = 11.7kWh
• Controlled Load = 498kWh / 91 days = 5.5kWh per day

Given that this property already has solar, as indicated by the ‘solar exports’, we can gauge when they’re using power. If they are exporting power to the grid, this means they must be at least covering daytime usage with the excess going to the grid, which indicates that their usage must be occurring overnight. 

Looking at the daily usage will indicate how much battery storage the homeowner needs, but we need to ensure there is enough excess energy to charge the batteries. Comparing daily usage (kWh) to daily export (kWh), we can determine whether there is sufficient excess power to charge a battery or if they need additional solar to cover a shortfall. 

Calculation:

• Average daily usage = 13.75kWh
• Average daily export = 11.7kWh

This property consumes 13.75kWh per day, and in the same period exports 11.7kWh to the grid. Which indicates that this consumption must be occurring overnight. We can see that they are exporting slightly less than they are consuming, but this is negligible. To calculate the battery size we would take the lesser of the two values, in this case 11.7kWh is the amount of storage that would be required. 

Example 3: You Use More Than You Export

• Average daily usage: 30kWh
  
• Average daily export: 10kWh
  

This home is using a lot more power than it’s sending back to the grid. With only 10kWh of excess solar on an average day, that’s all you have available to store in a battery.

Why This Limits Battery Size

A battery can only charge from surplus solar — unless you also charge from the grid. If you’re only exporting 10kWh a day:

• A 10kWh battery will fill on a sunny day, but that’s it.
  
• A 15–20kWh battery would rarely fill completely from solar, especially in winter. The extra capacity would sit underused.
  

That’s why, in many cases like this, we recommend adding more solar first before investing in a large battery — as explained in our blog Why Am I Still Getting an Electricity Bill?. More generation means more excess to store.

But What If You Can’t Add More Solar?

Sometimes adding panels just isn’t possible. You might be limited by roof space, shading, or a budget that can’t stretch to both solar and storage upgrades. In these cases, you have another option: force charging your battery from the grid.

How Force Charging Works

Some energy plans let you charge your battery from the grid during specific low-cost or free periods — then use that stored power when rates are higher. For example, OVO Energy’s “Free 3” plan gives you three hours of free electricity each day. If your battery can be charged in that window, you can fill it without relying solely on solar exports.

This approach can:

• Reduce your peak-time grid purchases
  
• Let you run a larger battery even with low solar exports
  
• Take advantage of free or discounted energy periods
  

The Trade-Offs

• You’re still dependent on the grid — so if it goes down, you won’t be able to force charge.
  
• It works best if you’re disciplined about timing your charging to those free/cheap periods.
  
• If your plan changes or incentives drop, the savings could reduce.
  

Key Takeaway

If you use more than you export, and adding solar isn’t an option, a time-based plan with force charging could be a viable way to make a battery work harder for you. But you’ll need to run the numbers to make sure it’s worth it — and choose a plan that suits your habits.

When Bigger Batteries Might Be Worth It

Not every home needs a large battery — but for some, going bigger offers real benefits beyond just storing solar. If you care about blackout protection, long-term flexibility, or joining a Virtual Power Plant (VPP), it might be worth sizing up.

Here are three reasons to consider it.

1. You Want Better Backup Power During Blackouts

Most batteries let you set a backup reserve — a percentage of stored energy that’s kept aside in case of a power outage. If your reserve is only 10%, a 10kWh battery leaves you with just 1kWh during an outage.

But with a larger battery, you can set a higher reserve without sacrificing too much usable storage.

Example:
A 24kWh battery with a 40% reserve leaves you with 9.6kWh for outages — compared to just 2.4kWh on a 10% reserve.

If your area gets frequent blackouts, or you rely on medical devices, a bit of extra storage could be the difference between being covered and caught short.

2. You Want to Join a Virtual Power Plant (VPP)

Some energy retailers run Virtual Power Plant (VPP) programs, where you allow them to draw from your battery during peak demand. In return, you’re paid — often at rates well above standard feed-in tariffs — and in some cases you’ll also get a sign-up incentive.

This can be quite fruitful if you have surplus battery storage. When you’ve got plenty of energy left after covering your own needs, it usually doesn’t bother you if the retailer takes a small amount — especially when you’re being well compensated for it.

The risk comes when your battery is sized tightly to your usage. If the VPP draws power you were counting on for the evening, you might end up having to buy electricity back from the grid at peak rates — which can undo some of the financial benefits.

VPPs can be worth exploring if you regularly have excess stored energy, or if you’re sizing up your battery with participation in mind. For more on how these programs work, check out Amber’s SmartShift VPP.

3. Your Usage Might Increase Soon

Planning on:

• Adding an EV charger?
  
• Installing ducted air con?
  
• Renting out a room?
  

If your household energy use is likely to grow, a slightly larger battery now might save you from having to expand later — especially with the rebate only claimable once (we’ll cover that next).

When Smaller Batteries Are a Smarter Fit

Not everyone needs the biggest battery on the block. In fact, for many homeowners, a smaller battery that’s well-matched to their usage offers better value — with fewer complications and a faster return on investment.

Here’s when it makes sense to keep things simple.

Your Usage Is Stable

If your household isn’t changing anytime soon — no EVs, no big appliances, no new tenants — a right-sized battery based on your current usage is usually enough. There’s no need to future-proof if your future looks the same.

You’re Not Interested in VPPs

If the idea of letting your retailer tap into your battery doesn’t appeal to you, you won’t need the extra headroom that larger systems offer for energy trading. Stick with a size that covers your needs and nothing more.

You Don’t Get Blackouts (Or Don’t Care About Backup)

If your area has a stable grid and you’re not fussed about keeping the lights on during the occasional outage, a small reserve — or none at all — is fine. No need to oversize for something that rarely happens.

You Want to Maximise Savings, Not Storage

Smaller batteries generally cost less and pay for themselves sooner — especially if you’re already exporting more solar than you use at night. In that case, even a 10kWh battery could make a noticeable dent in your bill.

But there’s one thing you need to keep in mind, no matter what size you’re considering — and that’s the rebate.

You Only Get the Rebate Once — So Size Carefully

Here’s something most people don’t realise until it’s too late: the federal battery rebate can only be claimed once.

That means if you install a 10kWh battery now, and later decide you should’ve gone with a 15kWh system — you’ll be paying full price for the upgrade. No second rebate. No top-up discount.

Why This Matters

It’s easy to underestimate your usage, especially if your needs grow over time. Maybe you:

• Start working from home
  
• Buy an EV
  
• Install electric heating or cooling
  
• Get hit with longer blackouts
  

If your battery can’t keep up, you’ll be stuck either buying from the grid (at peak prices) — or paying thousands to expand your storage without any incentive help.

The Safer Play

If you’ve got strong solar exports and you think your usage might increase, it may be smarter to size up slightly now — while the rebate still applies.

For more on how the rebate works — and why it’s changing — check out our guide:
Government Energy Rebates Drop in December 2025: Complete Guide

Final Checklist: How to Choose a Battery Size That Works for You

Still weighing up your options? Here’s a straightforward checklist to help you make sense of the numbers — and feel confident about your decision.

Do You Already Have Solar?

Yes:
→ Check your bill for both average daily usage and average daily export.
→ A healthy export number on its own doesn’t guarantee you can charge a battery — you need to compare it against your usage patterns.
→ Use the calculation we covered earlier: your battery size should not exceed your average daily export. If your export is less than the battery capacity you’re considering, it won’t fill consistently, especially in winter.
→ If exports are low compared to usage, adding more panels before storage will usually give you a better return.

No:
→ Start with your average daily usage from your bill.
→ Apply the 50–75% rule of thumb to estimate battery size, depending on how much of your usage happens at night.
→ Make sure your planned solar system is big enough to power your home and charge your battery, using the solar sizing formula from Section 2.

What’s Your Lifestyle Like?

• Home during the day?
  → You might not need a big battery — your solar already covers daytime usage.
  
• Out all day, home at night?
  → A larger battery may suit you better — more of your usage happens after sunset.

Do You Want Backup Power?

• Frequent blackouts or medical needs?
  → Go bigger and set a higher reserve level. (look for something with full home back-up like Sigenergy)
  
• Reliable grid and no outage concerns?
  → Smaller battery with minimal reserve may be fine.

Are You Joining a VPP?

• Yes:
  → Consider oversizing slightly to avoid running short during energy sharing events.
  
• No:
  → You can stick with a tighter match to your usage.
  

Are You Expecting Higher Usage in Future?

• New appliances, EV, extra rooms or tenants?
  → Plan for it now — remember the rebate is one-time only.

Still Not Sure?

Start with your bill. Talk to your installer. And don’t feel pressured to go bigger just because. The right battery size is the one that matches your needs, fits your budget, and makes the most of the solar you already have — or plan to install.