Sizing an off-grid system is the highest-stakes calculation in solar, because there’s no grid to forgive a mistake. Get it too small and you’ll sit in the dark or run the generator constantly through winter; get it too big and you’ve sunk tens of thousands of dollars into capacity you never use. Unlike a grid-tied system — where the grid quietly covers any shortfall — an off-grid system has to stand entirely on its own numbers. The good news is there’s a clear, logical method, and it works backwards: from how much power you use, through how many cloudy days you need to ride out, to the battery and the array.
Step 1 — Nail down your daily energy use
Everything starts here. You need to know, in kilowatt-hours, how much electricity you use in a day — and to be honest about it, because the whole system is built on this number. List your appliances and how long they run: fridge, lights, water pump, washing machine, devices, and any big loads like electric heating or hot water.
Off-grid also rewards reducing this number. Efficient appliances, LED lighting, gas or wood for cooking and heat, and solar hot water all shrink your daily demand — and because every kWh of demand drives the size (and cost) of everything else, cutting usage is the cheapest way to shrink the whole system. Off-grid design and energy efficiency go hand in hand.
Step 2 — Decide your days of autonomy
This is the off-grid-specific concept that grid-tied homes never deal with. Days of autonomy is how many days your battery bank can run the house with no solar input at all — through a run of dark, wet winter days. More autonomy means a bigger (more expensive) battery bank but more security; less autonomy means leaning on the generator more often.
Most New Zealand off-grid systems aim for somewhere around two to three days of autonomy, with the backup generator covering anything longer. This is a genuine trade-off: you’re balancing battery cost against how often you’re willing to run the generator. There’s no single right answer — it depends on your site’s winter sun and your tolerance for generator use.
Step 3 — Size the battery bank
Now you can size storage. The battery bank has to hold your daily usage multiplied by your days of autonomy — and then some, because you can’t drain batteries to empty without shortening their life. So the usable capacity must cover (daily use × autonomy days), and the total capacity is larger again to allow a safe depth of discharge. This is why off-grid battery banks are so much bigger than the optional battery on a grid-tied home: they’re carrying the whole house for days, not just shaving an evening peak.
Step 4 — Size the solar array
The array has to do two jobs at once: run the house during the day and fully recharge the battery bank — and it has to manage this on your location’s worst realistic solar days, not its best. That means sizing against winter sun-hours, which in much of New Zealand are far lower than summer. An array sized only for summer will leave you short and generator-dependent every winter. So off-grid arrays are deliberately generous, built to refill the batteries even when the days are short and grey.
Step 5 — Add the generator as the safety net
Finally, the backup generator covers the gap your batteries and array can’t — the genuinely long cloudy spells of deep winter. Sizing the generator and accepting some winter running is usually far cheaper than building a battery-and-array big enough to survive the worst week with zero generator use. The generator is the economical insurance policy that lets you size the rest of the system sensibly rather than for the absolute worst case.
Why this matters
Get this chain right and an off-grid home runs smoothly, with the generator rarely firing. Get it wrong — undersize the battery or the winter array — and you’ll either sit in the dark or run (and refuel) the generator far more than you bargained for. It’s genuinely specialist design work, which is why off-grid is a job for an experienced off-grid installer, not a scaled-up grid-tied quote.
The verdict
Off-grid sizing works backwards: start with your honest daily energy use, decide how many days of autonomy you want, size the battery bank to carry that load, size the array to refill it even in winter, and add a generator as the cost-effective safety net. Cutting your daily usage shrinks everything downstream, and sizing against winter — not summer — is what separates a system that works from one that leaves you in the dark. It’s precise, high-stakes work best left to an off-grid specialist.
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Sources: Off-grid sizing methodology, days-of-autonomy, and depth-of-discharge principles per NZ off-grid industry references (2026). Figures vary by site, season, and usage.