What size generator does an off grid house need?

An off grid house doesn't need a generator big enough to run the whole house. It needs one big enough to feed the inverter charger at its full charging rate, plus whatever runs in the house while the generator is on, with some headroom. For a common 5 kVA Victron MultiPlus-II, Victron's minimum recommended size is an 8 kVA generator. A generator much bigger than the charger can use costs more, is noisier and burns more fuel per kWh.

Why size the generator to the charger, not the house?

Off grid, the generator's main job is charging the battery through the inverter charger, which has a fixed top rate. Once the charger is flat out, a bigger generator can't push any more energy into the battery.

Here is how Victron's Australian manual rates the MultiPlus-II range, with our rough figure for power into the battery at its 57.6 V absorption voltage, before charger losses:

• MultiPlus-II 48/3000/35-32: 35 A, about 2 kW
• MultiPlus-II 48/5000/70-50: 70 A, about 4 kW
• MultiPlus-II 48/8000/110-100: 110 A, about 6.3 kW
• MultiPlus-II 48/10000/140-100: 140 A, about 8 kW
• MultiPlus-II 48/15000/200-100: 200 A, about 11.5 kW

The generator has to supply that charging power plus the charger's losses and whatever the house uses during the run. If the battery's management system accepts less current than the charger can give, the lower number decides how much generator you need.

How big should the generator be for a Victron system?

Victron's generator FAQ, last updated 28 March 2025, gives minimum sizes: a 5 kVA generator for a 3,000 VA inverter charger and 8 kVA for a 5,000 VA unit. It calls these safe minimums, because flat batteries and large loads together can still overload a generator.

Three reasons for that headroom:

• kVA is not kW. Victron's worked example says a 5 kVA generator typically gives about 4 kW.
• Duty cycle. Victron warns that some generators are marketed on peak output but are only built to hold about half of it for long periods. Battery charging is a long, heavy load.
• Inverter generators. Victron suggests choosing an inverter style generator at least 30% larger than an equivalent model with an automatic voltage regulator (AVR), because inverter generators are electronically limited and more easily overloaded by surges.

The inverter charger helps too. Its AC input current limit is set to match the generator, and Victron suggests starting at about 80% of the generator's capacity in watts. With PowerAssist on, the inverter tops up from the battery when the house briefly wants more than the limit, so the generator needn't cover the kettle, oven and bore pump at once. Victron notes that PowerAssist sets a minimum input current limit, though, and with a small generator it may need to be turned off.

Why does an oversized generator waste fuel?

The charger caps what the generator can deliver, so an engine far bigger than that spends its life running lightly loaded, and a lightly loaded engine burns far more fuel for each kWh it makes. A Victron white paper on generator running costs gives a typical 7.5 kW diesel using about 1.5 litres an hour at low load and 2.3 litres an hour at 75% load: only about half as much fuel again for a much bigger output.

Picture a 20 kVA diesel on a 5 kVA MultiPlus-II. The charger draws about 4 to 5 kW, around a quarter of what the engine was built for. The same white paper says fixed speed diesels last longer when run under load than at no or low load, and that some manufacturers recommend at least 30% load to prevent premature failure. You've also paid for a bigger machine and more noise.

When Victron says a bigger generator is usually better, they mean bigger than their minimums, and modest headroom is good. A generator two or three times bigger than the charger and house can use is where the waste comes in.

Do you need auto start on an off grid generator?

Yes, for a house you live in. With auto start, the system starts the generator when the battery drops to a set level and stops it once the battery recovers, so nobody has to watch it through a wet July week.

That means a generator with electric start and a remote start input, not a pull start machine. The MultiPlus-II has a programmable relay that can be set up as a starter relay for a generator. We build our off grid systems with Victron GX monitoring and the VRM portal, and we monitor the systems we install remotely from Trentham.

Set quiet hours too. Under the Environment Protection Regulations 2021, EPA Victoria groups equipment with an internal combustion engine with lawnmowers. That noise is unreasonable before 7 am or after 8 pm on weekdays, and before 9 am or after 8 pm on weekends and public holidays, if someone in a habitable room of another home can hear it. On a big rural block that may never matter; on a smaller one it should shape your settings.

Petrol, diesel or LPG: which fuel suits an off grid house?

For a home leaning on the generator every winter, diesel is the usual choice. LPG suits people who already have gas on site. Petrol suits light, occasional backup.

• Diesel: a fixed speed diesel with a good AVR alternator suits long runs under steady load, which is what battery charging is. But diesels dislike running lightly loaded, one more reason not to oversize.
• Petrol: many quiet inverter generators run on petrol. Check the duty cycle rating before trusting one with long charging runs, and allow the 30% extra size Victron suggests. Storing petrol near a house is a real fire risk.
• LPG: handy if you already have bottles or a tank for cooking. Check the generator's rated output on gas, not just its headline figure, and make sure your gas supply can keep up with a long, heavy run on a cold morning.

How many hours will the generator run in a Central Victorian winter?

That depends on the gap between what your array makes in winter and what the house uses, divided by the charger's rate. Generator size barely comes into it.

Say grey days leave the battery 16 kWh short. A 70 A charger puts in about 4 kW at full rate, so that's roughly four hours, a bit longer as charging tapers near full, whether the generator is 8 kVA or 20 kVA. To cut run hours you need a bigger charger, more panels or a bigger battery. When the array is sized for winter, most generator hours tend to fall in the short days of June and July. Our pieces on what panels make in a Central Victorian winter and how much battery an off grid house needs cover the inputs, and our free sizing tool runs the numbers for your house.

What about noise, fuel storage and safety?

Put the generator away from bedrooms and from any window or door it could blow exhaust through. Victron points out that charging is a heavy load and the installation needs plenty of airflow to get rid of the heat. Never run one in a closed shed or garage, because the carbon monoxide can kill. Store fuel away from the house and hot exhaust.

What an owner can safely do:

• Check oil, coolant and fuel levels with the generator stopped and auto start switched off
• Keep the air intake and enclosure clear of leaves, grass and nests
• Watch the battery in the VRM portal and log generator run hours and services

What needs a licensed electrician: anything inside the switchboard, the generator's connection to the inverter's AC input, changeover switches, the start wiring, and any change to the inverter's current limits or settings. Batteries and inverter chargers can deliver dangerous currents even with the generator off. Our generators page explains how we set up backup generators.