In an off-grid system, the battery bank isn’t an optional add-on the way it is on a grid-tied home — it’s the beating heart of the whole setup. It’s what carries your house through every night and every cloudy spell, standing in for the grid you don’t have. It’s also the single most expensive component, and the one whose health you most need to protect. So it’s worth understanding what an off-grid battery bank is, how the chemistries differ, and why one concept — depth of discharge — matters more than almost anything else.
Why the battery bank is so central
On a grid-tied home, the grid is your infinite battery: surplus goes out, shortfall comes in. Off-grid has none of that. Every unit of power you use after dark, and through every grey winter day, comes from the battery bank. If it’s too small or poorly looked after, you run out — and there’s no grid to top you up. That’s why off-grid battery banks are far larger than the evening-shaving battery on a grid-tied home, and why getting the battery right is the core of a good off-grid system.
The chemistries: lithium vs lead-acid
Two broad battery types are used off-grid, and the choice has largely settled:
- Lithium (typically LiFePO4). The modern standard. Lithium batteries last far longer (many more charge cycles), can be discharged much more deeply without harm, are lighter, need little maintenance, and charge more efficiently. They cost more upfront but usually win over the system’s life.
- Lead-acid (including AGM and gel). The traditional option — cheaper upfront and proven, but heavier, shorter-lived, less efficient, and they can’t be discharged as deeply without damage. Some still use them, especially on tighter budgets, but lithium has become the default for new off-grid systems.
For most new off-grid installs in New Zealand, lithium is the sensible choice — the longer life and deeper usable capacity generally justify the higher price.
The concept that matters most: depth of discharge
Here’s the idea that makes or breaks a battery bank’s life. Depth of discharge (DoD) is how much of a battery’s capacity you actually use before recharging. Drain a battery deeply and often, and you wear it out faster; keep it topped up and only lightly cycled, and it lasts far longer.
This is why you can’t just size a bank to your exact daily usage. If you need 10 kWh overnight, you don’t buy a 10 kWh battery and run it to empty every night — that would destroy it quickly. You buy more than you need, so each night uses only a portion of the capacity. Lithium tolerates deeper discharge than lead-acid (part of why it’s preferred), but the principle holds for both: usable capacity is always less than total capacity, and respecting that is what gets you a decade-plus of life rather than a few hard years.
Cycle life and replacement
A battery’s lifespan is measured in charge cycles — one cycle being a charge and a discharge. A quality lithium bank is rated for thousands of cycles, which translates to well over a decade of daily off-grid use. But no battery lasts forever, so an honest off-grid budget plans for an eventual bank replacement down the track. It’s the single biggest long-term cost of off-grid living, and worth factoring in from the start.
Sizing the bank
Sizing follows from your daily usage and your chosen days of autonomy (how long the bank must run the house with no sun). The bank’s usable capacity has to cover your daily use multiplied by those autonomy days, with total capacity larger again to keep depth of discharge healthy. That’s a calculation best done with an off-grid specialist — it’s covered in the off-grid sizing guide.
The verdict
The battery bank is the heart of an off-grid system: it carries your whole house overnight and through cloudy spells, it’s the biggest single cost, and its life depends on how well you respect depth of discharge. Lithium (LiFePO4) is now the default for its long life, deep usable capacity, and low maintenance; whatever the chemistry, always size for usable capacity below the total, plan for an eventual replacement, and size the bank together with your array and autonomy needs. Get the battery right and the rest of the off-grid system follows.
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Sources: Off-grid battery chemistry, depth-of-discharge, and cycle-life principles per NZ off-grid industry references (2026). Figures vary by product and usage.