Off-Grid Generator Guide: Sizing, Auto Start and Fuel

Nobody moves off-grid dreaming about a generator. You picture panels on the roof and a quiet battery in the shed. Then June arrives in the hinterland, the cloud sits on the range for a week, and the generator is the thing that keeps the fridge cold and the water pump running.

Most off-grid solar systems should have a backup generator. It covers long runs of cloudy weather and unusually heavy use, and it lets you size the battery for normal conditions instead of the worst week in a decade. The generator should start and stop automatically under the control of the off-grid inverter-charger, and it should be sized to run the house and charge the battery at the same time, within what the inverter-charger can accept. Get those three things right and a well-designed system only runs the generator occasionally, mostly in winter.

We design and integrate generators into off-grid systems across the Noosa hinterland and the Gympie region. Here's how we think about it.

Do you need a generator for off-grid solar?

Not strictly. But we recommend one for almost every full-time off-grid home, and the reason is money as much as reliability.

Without a generator, the battery has to carry you through the longest stretch of poor sun you're ever likely to see. That means buying a lot of storage that sits idle most of the year. With a generator, you size the battery for typical conditions (most homes we design land on 20 to 40 kWh for two to three days of autonomy) and let the generator handle the rare exceptions. Our guide to off-grid battery sizing walks through that trade-off with a worked example.

It also covers the weeks that aren't about weather: visitors, a building project, or a welder in the shed using more in a few days than the system was designed for. The generator is the insurance policy. A good design means you rarely claim on it.

How does off-grid generator auto start work?

The generator doesn't decide when to run. The off-grid inverter-charger does. It watches the battery and the loads, sends a start signal when conditions are met, synchronises with the generator, uses its power to run the house and charge the battery, then stops it when the job is done. You don't need to be home.

Both off-grid platforms we install do this, with slightly different menus.

Selectronic SP PRO. Selectronic's manual lists generator start and stop triggers based on battery state of charge, DC voltage, battery load, unit temperature, time and remote control. It supports remote-start generators in 2-wire or 3-wire configurations. It also works alongside the generator rather than just switching to it: when the site loads are greater than the generator's output, the SP PRO adds its own power, and when the loads drop it goes back to charging the battery, with no break in supply (Selectronic SP PRO manual). Our Selectronic SP PRO review covers the rest of the platform.

Victron. On a Victron system, generator control runs through a GX device such as the Cerbo GX, using its internal relay. Victron lists start and stop conditions including battery state of charge, battery current, battery voltage, AC load, inverter high temperature and inverter overload, plus a periodic run, quiet hours and a minimum run time (Victron GX generator auto start/stop). Victron's PowerAssist feature similarly lets the inverter-charger top up a generator's output from the battery (Victron). We cover the wider ecosystem in our Victron off-grid system guide.

The one requirement on the generator side: it needs electric remote start. A pull-start generator can't be automated. If you already own a generator, check this before you assume it can be integrated.

Setting the start and stop points

The trigger points are a design decision, not a factory default. Start too early and the generator runs when the sun would have caught up by lunchtime. Start too late and you cycle the battery deeper than needed. We set them from the battery manufacturer's guidance, your loads and how you feel about evening generator noise.

What size generator do I need for off-grid solar?

Size the generator for the job it actually does: running the house and charging the battery at the same time. Then check the result against what the inverter-charger can accept.

Here's the method. Every number below is illustrative, chosen to show the arithmetic, not a figure for your system. Your installer works from the inverter-charger's datasheet and your real loads.

A few things change that answer.

The inverter-charger has an input limit. Selectronic lists a maximum AC input source of 15 kVA for its SPMC models and 30 kVA for its SPLC models (Selectronic). A generator much bigger than the inverter-charger can use is wasted money.

A smaller generator can work, with compromises. Victron's own guidance is that the charger alone can overload a generator, that a bigger generator is usually better, and that if your generator is smaller than recommended you'll need to reduce the AC input current limit or the charge current to stop it overloading (Victron MultiPlus generator FAQ). The practical result is longer run times, because the battery charges more slowly.

Generators like to work. Selectronic's manual recommends loading the generator to the highest possible level while it's running and supplying sustained large loads directly from it. A big generator ticking over at a light load for hours is inefficient. So when the generator is on, that's the time to run the washing machine, the dishwasher or the water pump.

Motor loads surge. Queensland's Electrical Safety Office notes that motor-operated appliances like air conditioners, refrigerators and freezers can take two to 10 times their rated load when starting up (Electrical Safety Office). On a well-integrated off-grid system, the inverter-charger and battery help absorb those surges, which is one reason a generator wired through the inverter-charger copes better than one running the house on its own.

kVA and kW aren't the same. Generators are often rated in kVA and loads in kW, so make sure your installer and dealer compare like with like.

Inverter generator vs standard generator for off-grid

"Inverter generator" sounds like a natural match for an off-grid inverter. It isn't that simple. Victron explains that generator power is typically more variable than grid power, and an inverter-charger will reject power outside its programmed limits, so it won't connect, synchronise or charge. Settings can be widened for a less stable generator, at some cost to efficiency or transfer time (Victron MultiPlus generator FAQ). On the two generator types, Victron's guidance:

• Conventional generators with an automatic voltage regulator (AVR) audibly struggle when overloaded, and cope better with a brief high-current load.
• Inverter-based generators are electronically limited, so they're more easily overloaded by surges. They can also keep the motor running after they've electronically overloaded and stopped delivering power, so the system looks like it's charging when it isn't. Victron suggests that if you want an inverter generator (for example, for noise at lower loads), get one at least 30% larger than the equivalent AVR model.

Neither is automatically right. The generator has to be matched to the inverter-charger, tested together on site, and the settings adjusted to suit.

Diesel vs petrol generator for off-grid

• Diesel is usually chosen for serious running hours, such as a full-time home with a big winter load or a farm. It's generally heavier and dearer upfront.
• Petrol is generally cheaper and lighter, which suits a cabin or weekender where the generator rarely runs. But petrol doesn't keep well in storage.

Either way, it needs electric remote start and local servicing. Tell your generator dealer it will be integrated with an off-grid inverter-charger.

How often does an off-grid generator run?

On a well-designed system, not often. Most of the year the array and battery carry the house on their own. The generator mostly earns its keep in the winter design month, when days are short, and during long overcast spells.

How often yours runs comes down to design choices: how big the array is relative to winter use (a generous array recharges faster after cloudy days), how many days of autonomy the battery carries, and where the start point is set. If your generator runs every few days through spring and summer, your use has grown, a component isn't performing, or the system was undersized. That's worth a service call.

It's also worth exercising a generator you rarely need. Victron's GX includes a periodic run option for exactly this. A generator that sits for months is the one that won't start on the night you need it.

Generator servicing and run hours

Your monitoring usually records generator run hours, and servicing is normally based on hours run under the generator manufacturer's schedule. Your generator dealer handles the machine: servicing, fresh fuel (petrol especially doesn't keep), a healthy starter battery for auto start, and a clean, dry enclosure. On our side, an off-grid service checks that the inverter-charger still starts, synchronises with and stops the generator correctly, and that the start and stop points still suit how you live.

Where should an off-grid generator go?

Outside, always. Queensland's Electrical Safety Office says to never use a generator indoors, to install a battery-operated carbon monoxide alarm, to keep it dry under an open canopy-like structure, and to store fuel in labelled non-glass safety containers away from the home (Electrical Safety Office). On hinterland blocks, put it away from bedrooms, close enough to the inverter-charger for sensible cable runs, and somewhere you can refuel in the rain. Near the coast, ask your dealer about protection against salt air and corrosion.

Wiring a generator into the house in Queensland

Connecting a generator to a building's fixed wiring is licensed electrical work. The Electrical Safety Office says a licensed electrician must connect a generator to house circuits, references AS/NZS 3010 (Electrical installations: generating sets) as the relevant standard, and warns that plugging a generator into a wall outlet to "backfeed" the house is extremely dangerous (Electrical Safety Office). On an off-grid system, the generator connects through the inverter-charger's AC input, and that design is part of the installation.

How we integrate a generator into an off-grid system

Your generator dealer supplies and services the machine. Our job is the integration: specifying what the generator needs, wiring it into the inverter-charger, setting the start and stop logic, and testing it under load. If you already own one, tell us the make and model when we quote and we'll check it has remote start and is big and stable enough. If not, we'll give you the rating and features to ask your dealer for. Start with our off-grid solar systems page, or our guide to off-grid inverters if you're still choosing the platform.

Frequently asked questions

Do I need a generator with off-grid solar?

We recommend one for most full-time off-grid homes. It covers long cloudy spells and unusually heavy use, and lets you size the battery for normal conditions rather than the worst week you'll ever see. That usually costs far less than adding enough battery to go without one.

What size generator do I need for off-grid solar?

Add the power the inverter-charger draws at its full charge rate to the house load likely to run at the same time, then allow some headroom. Check the result against the inverter-charger's maximum AC input. A smaller generator can work if the charge current is turned down, but it will run for longer.

Can an off-grid generator start automatically?

Yes, if it has electric remote start. The off-grid inverter-charger starts and stops it based on conditions such as battery state of charge, battery voltage or load. Both Selectronic SP PRO and Victron systems (through a GX device) support this.

Is an inverter generator better for off-grid solar?

Not necessarily. Victron notes inverter generators are more easily overloaded by surges and can keep running without delivering power after an electronic overload. It suggests an inverter generator should be at least 30% larger than an equivalent conventional AVR generator if you choose one, usually for its lower noise.

How often should an off-grid generator run?

On a well-designed system, mainly in winter and during long cloudy spells. If it's running every few days in spring or summer, your use has grown, something isn't performing, or the system is undersized. A periodic exercise run also keeps a rarely used generator reliable.