You've got solar panels on your roof. Now you want a battery.
The question is: what size batteries for your solar panel?
Too small, and your battery runs flat before midnight. Too large, and your panels can never reliably fill it, meaning you're paying for storage that sits idle.
The good news is that your existing PV system is the best starting point for working out your ideal solar battery size.
Your Solar System Size Is the Primary Constraint
Before daily usage, before budget, before anything else: the size of your solar panel system sets the ceiling on how large a battery makes sense.
A solar battery can only store what your panels can generate. If you pair a large battery with a small solar array, it will rarely reach full charge, reducing both its efficiency and your return on investment.
How to calculate solar battery size based on your solar system?
The general rule in the industry is 1.2 kW of solar for every 2 kWh of battery capacity.
So for example, a 6.6 kW system is what's typically needed to reliably fully charge a 10+ kWh battery. This is a sizing ratio rule for solar PV systems, but it can be used in reverse to understand what's a good battery bank size fit for your household. The goal is straightforward here with your battery bank: reach full charge on a typical sunny day and then carry your household through the night.
Everything else follows from that.
Then Factor In Your Nighttime Usage
The Quick Way: Estimate From Your Bill
Once you know your system size, nighttime usage is the next number to nail down.
Your panels cover the day.
Whatever you draw from the grid between sunset and the following morning is what the battery needs to replace.
Check your electricity bill for your daily kWh average, then estimate how much of that falls outside daylight hours.
18kWh/day household average estimates:
- 7-9kWh nighttime demand
- Battery with 8-10kWh range for basic coverage
For households where most people are out during the day, roughly 40-50% of daily consumption happens overnight.
On an 18 kWh/day average, that's around 7-9 kWh of nighttime demand, pointing to a battery in the 8-10 kWh range for basic overnight coverage. If you run high-draw appliances in the evening, including ducted air conditioning, an EV charger, or electric water heating, that figure climbs quickly.
For many people, this estimate is enough to start a useful conversation with an installer. If you'd rather nail the number down more precisely before that conversation, the next section shows you how.
The Detailed Way: Calculate Your Evening Load Appliance by Appliance
If you want a more accurate figure than a bill estimate can give you, it's worth going through your evening appliance use step by step. This is especially useful if your usage pattern is irregular, or if you run high-draw appliances that can skew a simple percentage estimate.
Step 1: Find the Wattage of Each Appliance
Every appliance you use after dark has a wattage. Many have it printed directly on the unit. If yours only shows volts and amps, the calculation is simple:
Watts = Volts x Amps
So an appliance rated at 240V and 5A draws 1,200 watts. Once you have the wattage for each device, you have everything you need.
Step 2: Multiply by Hours of Use
For each appliance, multiply its wattage by how many hours you run it in a typical evening.
A few examples:
- 10 LED downlights at 10W each = 100W total. Run for 4 hours = 400 Wh
- A 200W TV (60 inch LCD) running for 3 hours = 600 Wh
- A 2,400W split system air conditioner running 2 hours (but only cycling about 50% of that time) = 2,400 Wh
Add these up to get your total watt-hours (Wh) of evening demand.
Divide by 1,000 to convert to kilowatt-hours (kWh), which is how battery capacity is measured.
A quick note on appliances that cycle on and off, like fridges, air conditioners, and pool pumps: they don't run at full draw continuously. As a rule of thumb, count them at roughly 50% of their rated wattage over the hours they're switched on. It's better to overestimate slightly than to run short.
Step 3: Add a Buffer
Once you have your evening total, add around 20% on top as a buffer because real-world usage is never perfectly predictable. For example, a cold night might mean the heater runs longer. A guest staying over adds load you didn't account for. Building in headroom also protects your battery from running to empty regularly, which extends its lifespan.
So if your evening calculation comes out at 8kWh, you're looking at a target solar battery size of around 9.5-10 kWh.
Battery Sizing by PV System Size
Here's how solar battery size recommendations land for the most common residential solar systems in Australia.
What Size Battery for a 5 kW Solar System?
A 5 kW system typically produces 17-22 kWh per day depending on location and orientation. Most households on this system size will find a battery in the 10-13.5 kWh range to be the right fit.
If you self-consume around 50% of production during the day, ~10 kWh works well. Alpha T10 or Alpha G3 both offer great options in 10kWh capacity.
If most of your output is being exported back to the grid, a 13.5 kWh battery captures more of that surplus. The Tesla Powerwall 3 (or Alpha ESS Smile 13.3 with 13.3kWh capacity) is a great match.
Our 10 - 13.5kWh battery range
- Alpha ESS T10 (10kWh)
- Sigenergy SigenStor 5.0 (10kWh with 2 modules)
- Anker Solix X1 (10kWh with 2 modules)
- Growatt ALP LV (10kWh with 2 modules)
- Alpha G3 (10.1kWh)
- Ambrion Raybox HS2 (10.24kWh)
- Alpha B3 Plus (10.08kWh with 2 modules)
- Alpha ESS Smile 13.3 (13.3kWh)
- Tesla Powerwall 3 (13.5kWh)
Learn more about being a 10kWh battery owner: 10kWh Battery Buying Guide
What Size Battery for a 6.6 kW Solar System?
The 6.6 kW system is one of the most common setups in Australia. It pairs well with batteries in the 10-15 kWh range for financial returns, or up to 16-20 kWh if energy independence is the priority.
For our range of 10 - 13.5kWh solar batteries, see above.
Our 14 - 20kWh battery range
- Fox ESS EQ4800 (13.94kWh in 3 modules)
- Sigenergy SigenStor 5.0 (15kWh in 3 modules, or 20kWh in 4 modules)
- Sigenergy SigenStor 8.0 (16kWh in 2 modules)
- Anker Solix X1 (15kWh in 3 modules, or 20kWh in 4 modules)
- Growatt ALP LV (15kWh in 3 modules, or 20kWh in 4 modules)
- Alpha ESS B3 Plus (15.08kWh in 3 modules, or 20.16kWh in 4 modules)
- Alpha ESS T10 (20.2kWh in 2 modules)
- Alpha G3 (20.2kWh in 2 modules)
- Ambrion Raybox HS2 (20.48kWh in 2 modules)
- Eveready Energy Vault (20.4kWh)
Learn more about what to expect with owning a bigger battery:
- 14kWh Battery Buying Guide
- 16kWh Battery Buying Guide
- The Federal Rebate's New Tiered System Come May 2026
What Size Battery for a 10 kW Solar System?
A 10 kW system suits higher-consumption households using 25-30+ kWh per day. Recommended battery sizes:
- For financial returns: 18-23 kWh
- For energy independence: 40 kWh+ (around 1.5 days of autonomy)
For our range of 18 - 20kWh solar batteries, see above.
Find 24 - 30kWh solar batteries listed below.
What Size Battery for a 13 kW Solar System?
A 13 kW system is typically found on larger homes or all-electric households. Solar battery bank sizes in the 24-50 kWh range are appropriate, with the upper end suited to households running EVs, ducted air con, and other high-draw loads.
Our 24 - 30kWh battery range
- Fox ESS EQ4800 (23.3kWh with 5 modules, or 27.96kWh with 6 modules)
- Alpha ESS B3 Plus (25.2kWh with 5 modules, or 30.24kWh with 6 modules)
- Anker Solix X1 (25kWh with 5 modules, or 30kWh with 6 modules)
- Sigenergy SigenStor 5.0 (25kWh with 5 modules, or 30kWh with 6 modules)
- Sigenergy SigenStor 8.0 (24kWh with 3 modules, or 32kWh with 4 modules)
- Tesla Powerwall 3 (27kWh with one expansion pack)
- Growatt ALP LV (30kWh with 6 modules)
- Alpha ESS T10 (30.3kWh with 3 modules)
- Alpha G3 (30.3kWh with 3 modules)
Our 35 - 50kWh battery range
- Alpha ESS Smile 13.3 (39.3kWh with 3 modules, or 53.2kWh with 4 modules)
- Fox ESS EQ4800 (41.93kWh with 9 modules)
- Alpha ESS T10 (40kWh with 4 modules, or 50kWh with 5 modules)
- Growatt ALP LV (40kWh with 8 modules)
- Sigenergy SigenStor 8.0 (40kWh with 5 modules, or 48kWh with 6 modules)
- Tesla Powerwall 3 (40.5kWh with 3 modules, or 54kWh with 4 modules)
- Alpha G3 (40.4kWh with 4 modules, or 50.5kWh with 5 modules)
Solar Battery Sizes Chart
So you can get an understanding of general solar battery sizing recommendations at a glance.
| Daily Usage | PV System | Battery for Returns | Battery for Independence |
10-12 kWh/day |
5 kW |
10 kWh |
13.5 kWh |
14-18 kWh/day |
6.6 kW |
10-13 kWh |
16-20 kWh |
20-25 kWh/day |
10 kW |
15-18 kWh |
32-40 kWh |
26-35 kWh/day |
13 kW |
20-23 kWh |
40-50 kWh |
These are indicative figures based on north-facing panels at a 30° tilt. Your installer should model your specific roof orientation, location, and usage profile.
What You Want From the Battery Also Matters
Most people adding a battery to an existing solar system are after one of three things, and each shifts the sizing differently.
To maximise solar savings: Size to what the solar system can reliably fill at least 60% of days. Smaller batteries tend to deliver better payback periods. Expecting to buy some grid electricity at certain times of year is normal and financially sensible.
For greater energy independence: You'll size larger than what's optimal purely for returns, accepting a longer payback in exchange for more autonomy. This makes the most sense in areas with unreliable grid supply or expensive peak tariffs.
For backup power: If you want to cover multi-day outages, you'll need significantly more capacity. Three days of autonomy is a good baseline for genuine resilience; five days is considered strong off-grid readiness. Bear in mind that systems designed for this purpose will have considerably longer payback periods.
Learn more about blackout protection.
Otherwise? Get What You Can Afford Now, Then Scale Up
If your ideal battery size is out of budget right now, that's okay. Start with what you can afford and choose a modular battery system such as Alpha ESS, Sigenergy or Fox ESS, which allow you to add capacity over time. Even a smaller battery that covers part of your evening load is delivering real savings from day one.
What you want to avoid is buying a closed, fixed-capacity system that can't be expanded, and then finding yourself locked out of growth later.
One important caveat: the federal Cheaper Home Batteries Program rebate can only be claimed once per property or premises.
Important Note: This also applies to properties you buy, rent or inherit. If a solar battery has claimed STCs with an installation at a dwelling, you won't be able to use the rebate to add on another battery.
This means if you undersize significantly now, any future capacity upgrade won't attract the same discount. If your budget allows you to get close to your target size from the start, that's the better financial move.
Where possible, also size slightly ahead of your current needs.
For instance, many people are considering electric vehicles to reduce their fuel and maintenance costs. However, an EV can also increase a household's energy use (and therefore bill) by around 30-50%.
A pool pump, additional air conditioning, or a growing household will also push demand up over time. That little headroom now can help save money in the long term, over retrofitting new equipment onto your existing system later.
Should You Add More Solar Panels at the Same Time?
Of course, if your existing system is undersized relative to your consumption, you might also want to add panels to improve your whole setup.
More generation means more energy available for your home and to fill up the battery each day.
Consider adding panels if:
- Your system regularly generates less than you consume during the day
- Your roof has available space and suitable orientation
- You're adding an EV or other high-draw appliance
What Does a Solar Battery Cost in Australia per kWh?
With the federal Cheaper Home Batteries Program in effect from 1 July 2025, upfront battery costs have dropped considerably.
- Pre-rebate pricing for lithium-ion systems (including inverter, software, and installation) has historically been around $850-$1,000+ per kWh
- The rebate is capped at approximately $244 per kWh of usable capacity up to 14 kWh from May 1 2026 with a reduced rate for every kWh after that, before admin fees (or around $335 per kWh and $300 after admin fees, before the May deadline)
- This represents an estimated 30% reduction in upfront installation cost
- A typical 10 kWh battery added to an existing system will come in around $8,000-$10,000; a 13.5 kWh battery sits around $10,000-$12,000
- Adding to an existing system costs a little more than buying new panels and a battery together
Key rebate eligibility requirements:
- Minimum 5 kWh usable capacity
- CEC-approved battery and inverter, installed by an accredited installer
- Must have VPP (Virtual Power Plant) capability, though joining a VPP is optional
- Can only be claimed once per property, so size carefully from the start
The Clean Energy Regulator now lets you get an estimate on the STCs (small-scale technology certificates) your installation may produce, which influences how much of the rebate you get to tap into. It's based on your location, how much usable battery capacity you are installing, and the date you have it installed.
STCs are what installers apply for, create, and sell on a marketplace to specific organisations that are required to offset their carbon (such as airlines). This way, installers handle the paperwork and you get an upfront discount.
Takeaways for Your Ideal Battery Fit
By now, you should know how to work out what size solar battery you need.
Your PV system size, your evening usage, and your goals are the three things that determine your ideal battery size. Get those right, and the rest follows.
- Start with your solar system size to set the upper limit on battery capacity
- Estimate from your bill for a quick starting figure, or calculate appliance by appliance for precision
- Add a 20% buffer either way, then cross-check against your system size
- Let your goals (savings, independence, or backup) determine where in that range you land
- Think ahead so you don't need a costly upgrade outside the rebate window
- Start with what you can afford if budget is tight, but choose a system you can expand
Want a Personalised Recommendation?
Get a recommendation for a battery sized to your existing solar system, with an estimate on how much you can be saving based on your actual usage.
