By Dave Miller · off-grid sparky · 22 years wiring solar, batteries and sheds in central QLD
“After losing half my lead-acid bank to Queensland humidity in a shed I thought was dry, I now insist on hermetically sealed battery boxes and rainwater tanks that actually drain.”
Sizing an off-grid battery bank comes down to one decision: how much energy you need to move through a 24-hour cycle without the grid catching your mistakes. Get the battery bank wrong and the rest of the system — panels, inverter, charge controller — won't save you. Water storage follows the same engineering logic: match capacity to demand, plan for the worst week, and keep the components rated for the job.
This guide covers the core of an off-grid energy and water setup: how the battery bank works, what specs matter, how to size it, and where rainwater storage fits alongside it.
An off-grid solar system is a stand-alone power generation setup that produces and uses electricity independently of the public power grid. The four main components are solar panels, batteries, a solar charge controller, and a power inverter.
Because off-grid systems are entirely independent of the utility grid, owners are 100% responsible for their power needs — you cannot draw extra electricity from the utility company when production falls short. Components must be specifically rated for off-grid use, since most grid-tie inverters are not configured to connect to a battery bank.
Off-grid solar systems use deep cycle batteries designed to be discharged and recharged gradually. Solar batteries are typically sized to cover energy usage for one night and recharge from solar during the day, completing one charge/discharge cycle over a 24-hour period.
When selecting a battery, two specifications matter most:
Other specs to compare across banks:
| Spec | Why it matters | What to check |
|---|---|---|
| Chemistry (LiFePO4 / FLA / AGM / Gel) | Efficiency, cycle life, maintenance | Match to climate and cycling pattern |
| Cycle life | Long-term value | Compare at the same depth of discharge |
| Usable capacity (kWh) | How long you can run | Apply real DoD, not nameplate |
| Continuous / peak discharge (A) | Inverter surge handling | Must exceed largest motor start load |
| Charge temperature limits | Battery longevity | LiFePO4 BMS often blocks charge below 0 °C |
| Enclosure rating | Humidity and corrosion | Hermetically sealed boxes suit QLD humidity |
Designing an off-grid battery bank requires:
1. Determining total energy needs (kWh per day).
2. Identifying peak energy loads (W or VA, including motor starting surges).
3. Accounting for local climate conditions.
4. Sizing storage capacity correctly to avoid shortages during high-demand periods.
Solar output often decreases during winter, so adapting the system to seasonal variations helps maintain smooth operation. Off-grid systems must be carefully sized to meet all energy needs, as there is no backup power source to rely on when solar production is low or consumption is high.
A practical sequence:
1. Measure every load with a plug-in power meter over a full week, including idle draws (fridge cycle, internet router, standby losses).
2. List peak loads — well pumps, fridge compressors, induction cooktops. The inverter must handle the sum of simultaneously running peak loads.
3. Add autonomy days — the number of days the bank should cover with no solar input. One night is the minimum; two to three is common for QLD summer storms.
4. Apply depth of discharge — FLA banks are usually limited to 50% DoD; LiFePO4 can run to 80–90% without accelerated wear.
5. Size the solar array for worst-month Peak Sun Hours, not summer averages, so the bank fully recharges even in June.
Modern MPPT charge controllers allow standard, mass-produced 60/120 or 72/144 cell solar panels to be used in off-grid applications. This simplifies parts sourcing considerably.
| Panel format | Approx. size | Notes |
|---|---|---|
| 60-cell / 120-cell | 1.07 m × 1.68 m | 120-cell panels use half-cut cells, which are slightly more efficient and resistant to failure than 60-cell panels |
| 72-cell / 144-cell | 1.07 m × 1.98 m | Higher voltage, lower current — useful for long cable runs |
Monocrystalline solar panels are slightly more efficient than polycrystalline panels, but both produce power equally well per panel; polycrystalline arrays take up more space. Choose mono when roof or ground-mount area is tight; poly when it isn't.
For inverters, pure sine wave output is appropriate for modern appliance electronics and fridge compressor loads. Match continuous rating to your measured load, and confirm surge capacity covers motor starting.
Off-grid solar battery systems offer benefits including:
The trade-off is full responsibility for sizing, maintenance, and backup — including a generator for extended cloudy periods when solar production cannot keep up with demand.
Rainwater harvesting pairs naturally with an off-grid power system because both rely on stored capacity to bridge gaps. Polyethylene tanks are appropriate for Australian rainwater harvesting and are the most common choice for rural properties.
Sizing follows the same logic as a battery bank: match storage to demand over the worst expected dry period, not the average.
Key design considerations:
For QLD properties, bushfire and UV exposure are real factors. Poly tanks handle both, but metal fittings, pumps, and exposed pipework benefit from shading or insulation.
Off-grid systems touch mains-equivalent AC voltage (230V at 50Hz in Australia), battery banks capable of delivering hundreds of amps, and rainwater plumbing that connects to potable fixtures. Treat each as serious work.
Call a licensed professional when:
DIY work is appropriate for: low-voltage DC wiring within manufacturer specs, mounting controllers and inverters on prepared surfaces, setting up rainwater tanks on level pads, and basic commissioning once a licensed sign-off is in place.
How long should an off-grid battery bank last?
Cycle life varies by chemistry and depth of discharge. Lithium iron phosphate banks typically outlast flooded lead-acid banks by several multiples when both are run at their recommended depth of discharge.
Do I need a generator if I have a battery bank?
Yes, in most climates. Extended cloudy periods can drain the bank faster than solar can refill it. A generator acts as insurance policy for those stretches. Make sure the changeover is wired through a proper transfer switch by a licensed electrician.
Can I use grid-tie solar panels on an off-grid system?
The panels themselves usually work, but the inverter almost never does. Off-grid systems need inverters specifically rated to connect to a battery bank, because most grid-tie inverters are not configured for that role. Modern MPPT charge controllers handle a wide range of standard panel formats.
How much water storage do I need?
Size for the longest dry spell you reasonably expect, not the average month. For a four-person household in Central Queensland, multi-thousand-litre poly tanks are typical. Match capacity to actual measured demand rather than rules of thumb.
What depth of discharge should I use?
It depends on chemistry. Flooded lead-acid banks generally stay healthier at 50% DoD or less. LiFePO4 handles 80–90% DoD routinely without the same wear penalty. Sizing should always apply the real usable figure, not the nameplate.
Can I expand the battery bank later?
Yes, if you plan for it from the start. Use the same battery model, similar age, and compatible configuration. Mixing old and new batteries in the same bank accelerates degradation of the older units.
Worth a watch: Off-Grid Solar Battery Bank and Water Storage with Martin Johnson Off-Grid Living · The Grass-fed Homestead
Grab a Kill-A-Watt meter and walk around the house plugging in each appliance you actually use. Log the watt-hours over a typical day, add a buffer for bad weather, and that gives you a real number to size to. It's a weekend job but it beats guessing and ending up with a bank that's too small.
Forget sizing for your average day — in central Queensland you design for the worst stretch, and a week of grey sky is realistic. I oversize the array so the bank can recover properly once the sun comes back. The article walks through how I work that out for my own place.
For the inverter, AC wiring and anything that ties back to mains, yes — get a licensed sparky in. I do my own DC interconnects and low-voltage work, but I'm qualified by trade. If you're not, the safe rule is anything that can kill you, leave it to someone who can sign the cert.
I had the same problem — lost half a lead-acid bank to humidity in a shed I was sure was dry. These days I only use hermetically sealed battery boxes and make sure the enclosure can breathe. Heat and moisture chew through batteries faster than normal cycling ever will.