It surprises a lot of people: nearly every off-grid solar home in New Zealand has a backup generator sitting in a shed somewhere. If the whole point of off-grid is independence from fossil fuels and the grid, isn’t a generator a contradiction? Not really — it’s the pragmatic safety net that makes off-grid affordable and reliable. Understanding why it’s there, and how to keep it running as little as possible, is part of doing off-grid well. Here’s the honest picture.
Why off-grid systems need a generator
The issue is winter. Off-grid systems are sized to run on solar and stored battery power, but New Zealand winters bring long stretches of short, dark, wet days when the panels simply can’t keep the batteries full. You have two ways to cover those spells:
- Build a huge battery bank and array — big enough to survive the very worst week of winter with no help. Possible, but extraordinarily expensive, because you’re sizing the most costly parts of the system for a situation that arises only a few weeks a year.
- Build a sensible battery bank and array for normal conditions, and add a backup generator to cover the rare long gloomy spells.
Option 2 is almost always far cheaper, which is why nearly every off-grid system uses it. The generator is insurance — it lets you size the expensive solar-and-battery side for typical conditions rather than the absolute worst case, and steps in only when needed.
How it fits into the system
In a well-designed off-grid home, the generator runs rarely — it’s not a daily workhorse. The batteries and panels handle the everyday; the generator only fires up when a prolonged cloudy spell threatens to drain the batteries below a safe level. Modern off-grid inverters can start the generator automatically when the battery state of charge drops to a set point, run it just long enough to recharge, then shut it down — so you may not even be involved. A good system might run the generator only a handful of times across the depths of winter.
Sizing and fuel
A backup generator is sized to charge the battery bank and run the house at the same time — big enough to do real work in a hurry, but not vastly oversized. As for fuel, the common options in New Zealand are:
- Diesel — efficient and durable, common for larger or full-time off-grid homes.
- Petrol — cheaper to buy, fine for smaller systems and occasional use.
- LPG — clean-burning and easy to store, used in some setups.
The right choice depends on system size, how often it’ll run, and what’s practical to store and resupply at a remote property.
How to keep it running as little as possible
Since generator running means fuel, noise, and maintenance, the goal is to minimise it. The levers:
- Size the solar and battery sensibly so the generator is genuinely a backup, not a crutch propping up an undersized system.
- Reduce winter demand — shift big loads to sunny days, use wood or gas for heating and cooking, and be a bit more frugal with power in the depths of winter.
- Maintain the batteries well, so they hold their capacity and need topping up less often.
- Keep the generator serviced so that when it does run, it runs efficiently.
An off-grid system that’s well sized and well managed barely touches its generator most years; a poorly sized one runs (and refuels) it constantly — which is both costly and exactly the fossil-fuel dependence off-grid was meant to escape.
The verdict
A backup generator isn’t a failure of an off-grid system — it’s the smart economics that make one affordable. Rather than paying for a giant battery and array to survive the worst week of winter, you size the solar side for normal conditions and let a generator cover the rare long gloomy spells, often starting automatically and running only occasionally. Size the rest of the system well, trim winter demand, and the generator stays what it should be: a quiet safety net you rarely need.
Get a free assessment and we’ll design an off-grid system that barely touches its generator.
Sources: Off-grid generator sizing, auto-start, and fuel-type guidance per NZ off-grid industry references (2026). Figures vary by system and site.