Most Cairns homes need a solar battery between 10 and 13.5 kWh. That size covers the evening and overnight power a typical FNQ family uses, including the air con that runs half the night in summer. The exact size for your home comes from one number, and you already have it sitting in your Ergon data.
We install batteries every week across Cairns, Port Douglas, the northern beaches and the Atherton Tablelands. This guide shows you the same simple method we use when we size a system for a customer.
Quick answer
- Most Cairns homes: 10 to 13.5 kWh of usable battery storage
- Small home, no air con at night: 5 to 8 kWh
- Family home with air con in the bedrooms: 10 to 13.5 kWh
- Big home with a pool and ducted air: 16 to 20 kWh or more
- Cyclone season backup: add 3 to 5 kWh of headroom on top of your overnight use
- The federal rebate pays about $252 per usable kWh on the first 14 kWh, which makes 13.5 kWh the sweet spot for most homes
The one number that sets your battery size
Battery sizing sounds technical. It is not. You need one figure: how much grid power your home buys between sunset and sunrise.
That number is your minimum battery size.
If your home pulls 11 kWh from the grid overnight, then an 11 kWh battery, or the next size up, lets your solar carry you through the night instead of Ergon. Simple as that.
Here is where to find the number:
- If you already have solar with monitoring, open your inverter app and look at grid import between about 6pm and 6am. Our monitoring systems show this hour by hour, and we wrote a full guide to reading your Fronius Solar.web data if you want to dig in.
- If you have a smart meter but no solar, Ergon’s online portal shows your usage in half hour blocks. Add up the blocks between sunset and sunrise.
- If you only have your paper bill, it shows your average daily use. That is a starting point, but it does not split day from night. Our Cairns electricity bill guide walks through what each part of the bill means.
Check a few summer nights, not winter ones. Summer is when your battery works hardest, because that is when the air con runs.
How much does a Cairns home use overnight?
A typical Cairns home uses 8 to 15 kWh between sunset and sunrise in summer. That is a lot more than homes down south, and there is one reason: air con.
A single bedroom split system running through a hot, humid night can chew through several kWh on its own. Add the fridge, a few fans, the TV, phone chargers and maybe a pool pump on a timer, and the numbers climb fast. We covered this in detail in our post on running air conditioning on solar.
The wet season, November to April, is when overnight use peaks. It is also when solar output dips on cloudy days. Keep both facts in mind as you size your system. We will come back to them.
What do you want the battery to do?
Before you pick a size, pick a goal. There are three common ones, and each has a different answer.
Goal 1: Cut the power bill
Size the battery to match your overnight use. For most Cairns homes that lands between 10 and 13.5 kWh.
Here is why this works so well in regional Queensland. From 1 July 2026, the regional feed-in tariff is 6.006 cents per kWh. Grid power on Ergon tariff 11 costs roughly 30 to 35 cents per kWh. So every kWh you store and use yourself is worth about five times what you get for exporting it. We broke down those numbers in our post on the Queensland solar feed-in tariff.
A battery in the 10 to 13.5 kWh range soaks up your spare solar during the day and covers the evening peak, the overnight air con, and the morning kettle before the sun gets going. The Tesla Powerwall 3 sits right at 13.5 kWh usable, which is one reason it suits so many FNQ homes. You can compare it with the other batteries we install, including BYD, Sigenstor and Fronius.
Goal 2: Blackout backup for cyclone season
Cairns sits in cyclone region C. When a storm takes the grid down, a battery with backup circuits keeps the essentials running.
For backup, take your overnight number and add 3 to 5 kWh of headroom. That buffer matters because during an outage you cannot count on a full charge at sunset, and the outage might run longer than one night.
Just as important as size is what the backup circuits cover. At minimum, we wire in:
- the fridge and freezer
- some lights
- ceiling fans
- phone and radio charging
That combination keeps food safe and the family comfortable without draining the battery in a few hours. We wrote a full guide to setting up a solar battery for cyclone season, and you can read how one of our customers’ Fronius battery kept their home running when the street went dark.
Goal 3: Near total independence
Some customers want to buy almost nothing from Ergon. That takes 20 kWh or more of storage, and usually a stacked modular system.
This is where modular batteries like BYD and Sigenstor shine. You stack extra modules to reach the size you need, and you can add more later. We built exactly this kind of system for a family at Kewarra Beach, and you can see the details in our Kewarra Beach Fronius and BYD battery case study.
Going further, to full off grid, is a bigger step again. If that is where you are heading, start with our off grid solar cost guide for Cairns.
Why the rebate maths points to 13.5 kWh
The federal Cheaper Home Batteries Program started on 1 July 2025 and takes roughly 30 percent off the installed cost of a battery. From 1 May 2026 it became tiered:
- about $252 per usable kWh for the first 14 kWh
- about $151 per kWh from 14 to 28 kWh
- about $38 per kWh from 28 to 50 kWh
Read those tiers again. The first 14 kWh earn the strongest rate. Every kWh above 14 earns a lot less.
That is why 13.5 kWh is such a common landing spot. It sits just under the 14 kWh line, so the whole battery gets the full rate. A 13.5 kWh battery typically costs about $12,000 to $16,000 installed before the rebate, and the rebate takes a serious chunk out of that. Our 2026 battery rebate guide for Cairns explains how to claim it, and our FNQ battery cost guide covers what different sizes cost after the discount.
One thing to be clear on: the old Queensland Battery Booster scheme closed in May 2024. The federal program is the rebate that matters now.
If the upfront number is still a stretch, we offer payment plans through Handypay, so the battery can start cutting your bill while you pay it off.
Can a battery be too big?
Yes, and it costs you real money.
A battery only earns its keep when it cycles, filling with cheap solar during the day and emptying into your home at night. A 25 kWh battery on a house that only uses 10 kWh overnight sits half empty most of the year. In the wet season, when cloudy days shrink your solar output, that oversized battery may barely fill at all. You paid for storage you never use.
Undersizing hurts too, just in a different way. A 5 kWh battery on a family home with air con empties before midnight. From then until sunrise you are back on Ergon at 30 to 35 cents per kWh, which is exactly the cost the battery was meant to kill.
The fix for both problems is the same: size from your real overnight number, not from a guess or a sales pitch.
Will your panels actually fill the battery?
A battery is only as good as the solar array feeding it. The panels have to do two jobs at once: run the house during the day and fill the battery for the night.
Our rule of thumb: about 1.5 kW of panels for every 2 kWh of battery, on top of what the house itself needs. Cairns gets about 5 peak sun hours a day, which is generous, but the wet season cuts into that. We always build in margin so the battery still fills on a grey December day.
So a 13.5 kWh battery wants roughly 10 kW of panels behind it once you count the house load. If your roof or your current system is smaller than that, the battery will regularly go to bed half full.
If you already have solar, two checks come first. One, is the array big enough, and two, is it healthy. Our solar health check picks up tired panels and inverter faults before you spend money on storage, and our post on adding a battery to an existing solar system covers the retrofit options, including hybrid inverters. If you are starting from scratch, we design the panels and battery together as one residential solar system, which is the cleanest way to get the sizing right.
Worked examples for FNQ homes
Here are the three home types we quote most often around Cairns, with the battery range that fits each one.
| Home type | Typical overnight use | Battery size | Notes |
|---|---|---|---|
| Small home or couple, no air con at night | 4 to 7 kWh | 5 to 8 kWh | A smaller modular battery covers the fridge, fans and lights with room to spare |
| Family home, air con in bedrooms overnight | 8 to 13 kWh | 10 to 13.5 kWh | The most common Cairns setup. A 13.5 kWh unit like the Powerwall 3 fits the rebate sweet spot |
| Big home, pool plus ducted air | 14 to 20 kWh | 16 to 20 kWh or more | Usually a stacked modular system, sized with backup headroom for cyclone season |
If your home does not fit neatly into one row, that is normal. Pool pumps, sheds, home offices and EV chargers all shift the numbers. That is why we size from your actual usage data, not from a table.
FAQ
Is a 10 kWh or a 13.5 kWh battery better?
If your overnight use is under about 9 kWh, a 10 kWh battery does the job for less money. If you run air con through summer nights, the extra 3.5 kWh earns its keep almost every day, and the rebate pays about $252 per usable kWh on the whole battery either way. When in doubt, we size up to 13.5 kWh because summer overnight use in Cairns almost always grows.
What size battery do I need for blackout backup?
Take your normal overnight use and add 3 to 5 kWh of headroom. For most Cairns family homes that means 13.5 kWh or more. Make sure the backup circuits cover the fridge, some lights, fans and phone charging, because that is what matters when a cyclone takes the grid down.
Can I add more battery capacity later?
Yes, if you choose a modular system. BYD and Sigenstor batteries stack extra modules, so you can start at 10 kWh and grow to 20 kWh or more as your needs change. The Powerwall 3 is a fixed 13.5 kWh unit, but you can add a second one.
Is there a solar battery size calculator I can use?
The best calculator is your own data: grid import between sunset and sunrise on a few summer nights. That single number beats any online tool, because it reflects your air con, your fridge and your habits. Send us a recent bill or your monitoring data and we will do the maths for you at no cost.
Get a battery sized for your home, not a brochure
We are G-Solar & Electrical, based at 32 Magazine Street in Stratford. Grant Newman, our owner, is a fifth generation Cairns electrician and SAA accredited, and we are a Fronius Partner Plus dealer with 326 Google reviews at a 5.0 rating. You can see recent installs across FNQ in our work gallery.
Send us your Ergon bill or your monitoring data and we will tell you the battery size that actually fits your home, plus the rebate you can claim on it. Request a free quote here and we will get back to you with straight numbers, no pressure.