On a grid-connected house, getting the battery size a bit wrong costs you some savings. Off-grid, it costs you the lights. There's no network to fall back on at 2am in the middle of a wet week in July, so the battery and the array have to be designed for the worst stretch of weather you'll realistically see, not for an average day.
Most full-time off-grid homes we design need 20 to 40 kWh of battery storage, which covers two to three days of autonomy, meaning two to three days of normal use with little or no solar coming in. The right figure for you comes from your daily energy use in winter, the number of days of autonomy you want, a reserve you never plan to use, and whether there's a generator to cover the rare long stretch of bad weather. In practice, a well-designed off-grid system pairs a sensibly sized battery with a generously sized array and a generator, rather than trying to do everything with batteries.
Off-grid design has been a genuine specialty of ours for years, across the Noosa hinterland and the Gympie region. Here's the method we use.
How much battery storage do you need for off-grid solar?
The short version, from our off-grid solar work: most residential off-grid systems need 20 to 40 kWh of storage for two to three days of autonomy.
Where you land in that range depends on four things:
- How much energy you use per day, measured in the worst month, not the average.
- How many days of autonomy you want the battery to carry without meaningful solar.
- How much reserve you keep so the battery is never run flat.
- Whether you have a generator, and how readily you're willing to run it.
Here's how the system sizes we typically design line up:
| System | Typical array | Typical battery | Suits |
|---|---|---|---|
| Starter off-grid | 5 to 10 kW | 10 kWh | Cabins, tiny homes, basic needs |
| Complete home | 10 to 15 kW | 20 to 30 kWh, with generator integration | Full-time residences |
| High capacity | 15 to 30 kW-plus | 40 to 100 kWh, scalable | Large properties, farms, remote small businesses |
Step 1: Work out your daily energy use
Everything starts here. Get this wrong and the rest of the arithmetic doesn't matter.
If you're moving from a grid-connected house, your bills show your average daily usage. That's a starting point, but adjust it: winter use is often different from the annual average, and off-grid households usually change habits once every kilowatt-hour comes from their own system.
If it's a new build or a block with no history, list what you'll run and for how long. Energy in kWh is just power in watts × hours ÷ 1,000. For example, a 150 W device running for 10 hours uses 150 × 10 ÷ 1,000 = 1.5 kWh. Add up every appliance for a winter day and that's your daily figure.
The loads that wreck off-grid budgets are the big resistive and motor loads: electric heating, air conditioning, pool pumps and some cooking and hot water. They need careful thought and may need a larger system. Energy-efficient appliances make every part of an off-grid system smaller and cheaper.
Step 2: Pick your winter design month
This is the step that separates a properly designed off-grid system from a grid-tied system with the wire cut.
You size for the month where your solar production is lowest relative to your use. On the Sunshine Coast and in the hinterland, that's usually mid-winter, when days are shortest and the sun is lowest, and it can coincide with runs of overcast weather. Panels still produce in cloud, but at reduced output.
Designing for the average month gives you a system that works brilliantly in January and leaves you running the generator for weeks in June. Designing for the worst month gives you a system that has surplus most of the year, which is fine. In off-grid, surplus is the price of reliability.
Step 3: Choose your days of autonomy
Days of autonomy is how many days the battery can carry your normal use with little or no solar input.
- Two days is a common starting point for a home with a generator that starts automatically.
- Three days suits households that want to run the generator less, or are further from fuel and help.
- More than three days gets expensive fast. Past a point, a generator covers rare long spells far more cheaply than another 10 or 20 kWh of batteries that sit idle most of the year.
That's why the 20 to 40 kWh range for two to three days of autonomy covers most homes. It's the balance point between battery cost and generator hours.
Step 4: Keep a reserve
You don't design an off-grid battery to be run to empty. Running a battery flat regularly shortens its life, and if it's flat, the next cloudy morning starts from nothing. So the calculation includes a reserve: a share of the battery you never plan to use in normal operation. Your installer sets this based on the battery chemistry and the manufacturer's guidance.
Modern lithium iron phosphate (LiFePO4) batteries handle deep cycling far better than older chemistries and typically last 10 to 15 years, but a reserve still matters. Some older battery types also need occasional equalisation charging, which is part of off-grid maintenance.
Off-grid battery calculator: a worked example
Here's the arithmetic laid out end to end. Every input below is illustrative, chosen to show the method, not a figure for your property.
| Input (illustrative) | Value |
|---|---|
| Daily energy use in the winter design month | 12 kWh |
| Days of autonomy | 2 |
| Share of the battery used in normal operation (the rest is reserve) | 80% |
| Allowance for inverter and battery losses | 10% |
The working:
- Energy to cover: 12 kWh × 2 days = 24 kWh
- Add the reserve: 24 kWh ÷ 0.8 = 30 kWh of usable capacity
- Add losses: 30 kWh × 1.1 = 33 kWh
So this example household would be looking at roughly 33 kWh of usable battery, inside the 20 to 40 kWh band. Change any input and the answer moves: drop to one and a half days of autonomy with a reliable auto-start generator and it falls; add a reverse-cycle air conditioner running on winter evenings and it climbs.
That's a starting estimate, not a design. A real design also checks the inverter can supply your peak power and surge, and that the array can actually refill the battery.
How much solar do I need to be completely off-grid?
Enough to run your house on a winter day and put back what the battery gave up during the last cloudy spell. That second part is what makes off-grid arrays big.
The method, again with illustrative numbers only:
- Daily array output needed = daily use ÷ system efficiency. If the example household uses 12 kWh and we allow for losses of, say, 25% across the system, the array needs to deliver 12 ÷ 0.75 = 16 kWh a day in the design month.
- Array size = that daily figure ÷ the peak sun hours your site gets in the design month. If a site averaged, say, 4 peak sun hours in its worst month, that's 16 ÷ 4 = 4 kW as a bare minimum.
- Then oversize it. A bare-minimum array only keeps pace on an average winter day. It never catches up after a cloudy run, so the generator ends up doing the recharging. That's why our complete off-grid home systems typically carry 10 to 15 kW of panels even when the bare-minimum maths says less.
We use location-specific solar data for your site and orientation when we design, rather than a rule of thumb.
Do you need a generator with off-grid solar?
Not strictly, but we recommend one for almost every off-grid home.
A generator gives you redundancy through extended low-sun periods, extreme weather, or a week when the household uses far more than usual. We integrate it so it starts automatically when the battery drops to a set point, charges the battery back up, and shuts down. You don't have to be home, and you don't wake up to a dark house.
It also changes the economics of the battery. Without a generator, you'd need enough storage to ride out the longest bad-weather stretch you're ever likely to see. With one, you size the battery for typical conditions and let the generator handle the rare exceptions. A generator that runs a handful of times a year is far cheaper than batteries that sit idle most of the year.
Power vs energy: the inverter matters too
Battery capacity (kWh) is the size of the tank. Inverter rating (kW) is how fast you can draw from it. Off-grid, you need both right.
Starting a bore pump, a pressure pump or an air conditioner pulls a surge well above its running draw, and with no grid behind it, the inverter has to supply that surge on its own. An undersized inverter on a big battery still trips when the pump kicks in. This is why off-grid inverters like the Selectronic SP PRO are built the way they are. Our Selectronic SP PRO review explains what to look for.
What does it all cost?
Off-grid systems range from about $20,000 to $100,000-plus, depending on your energy needs, battery capacity and backup requirements. The battery is a large share of that, which is exactly why sizing it properly matters. For the full breakdown, including when off-grid beats paying for a grid connection, see our off-grid solar system cost in Queensland guide.
If you're still deciding whether off-grid living suits you, our guide to off-grid living on the Sunshine Coast covers the lifestyle side.
Frequently asked questions
How much battery do I need for off-grid solar?
Most full-time off-grid homes need 20 to 40 kWh of storage, which gives two to three days of autonomy. Your figure comes from your winter daily use, the days of autonomy you want, a reserve you don't plan to use, and whether a generator is part of the system.
How much solar do I need to be completely off-grid?
Enough to cover your daily use in the worst winter month and recharge the battery after cloudy days. That's well above the bare minimum for an average day. Full-time homes we design typically have 10 to 15 kW of panels with 20 to 30 kWh of battery and a generator.
What are days of autonomy?
The number of days your battery can supply your normal use with little or no solar coming in. Two to three days is typical for a home with a backup generator. Going much beyond that usually costs more than running a generator occasionally.
Do I need a generator with an off-grid system?
We recommend one for most off-grid homes. It covers extended bad weather and unusually high use, and lets you size the battery for normal conditions rather than the worst week in a decade. We set it up to start automatically when the battery drops to a set point.
How long do off-grid batteries last?
Modern LiFePO4 batteries typically last 10 to 15 years. Sizing with a reserve, so the battery isn't regularly run flat, and keeping it out of extreme heat both help it reach the long end of that range.
