Do I need a generator for an off-grid system?

At full-time scale, almost certainly. The last two days of autonomy are the most expensive and least used part of a battery bank, and no amount of storage covers a week of snow on the array. A generator handles the rare long gap for a fraction of what the equivalent battery costs.

What size generator do I need off-grid?

Size it to what your inverter/charger can draw, not to your household loads. If the charger accepts 40 amps at 120 volts, that is about 4,800 watts of demand, and the generator has to supply that continuously rather than as a surge. Oversizing wastes fuel because a lightly loaded engine is inefficient.

Propane or gasoline for an off-grid generator?

Propane, in most cases. It stores indefinitely, where gasoline can start degrading within about thirty days and gums a carburetor when a machine sits unused. Off-grid properties usually have a propane tank already. The trade is a lower power rating on propane than on gasoline from the same engine.

Is a tri-fuel conversion worth it off-grid?

Usually not, because the third fuel is natural gas and that requires a utility gas line. A property without utility electricity rarely has utility gas. If you do have gas service and want backup for a grid-connected house, that is a different setup and a different set of rules.

Generators for Off-Grid Systems

By Matt · Updated August 29, 2026

The short answer

Off-grid, a generator is a battery charger, not a house supply. Size it to what your inverter/charger can actually draw rather than to your loads, and expect a cheap open-frame generator to be refused outright by a modern inverter/charger on power quality. Propane is usually the right fuel, because it stores indefinitely and a generator that runs thirty hours a year spends most of its life sitting still.

Almost every long-running off-grid home has a generator, and that is an engineering conclusion rather than an admission of failure. Storage is expensive at the margin. The last two days of autonomy are the part of a bank you pay the most for and use the least.

This page is about a generator inside an off-grid system. If you have utility service and you want power during outages, that is a different problem with different rules, and it starts at choosing your backup type. Connecting any generator to a house that also has a utility connection is transfer-switch territory, and getting it wrong can energize the utility line and kill a lineworker.

The generator charges the bank

This is the idea that reorders everything else.

Off-grid, you do not run the house from the generator. You run the house from the battery, and the generator’s job is to put charge back into the battery when the sun has not. It feeds the inverter/charger, which feeds the bank, and your loads never notice the difference.

That matters for three reasons.

The generator runs at a steady, known load. Charging is a flat demand, not the spiky profile of a house. Engines are happier and more efficient this way.

It can run for a couple of hours instead of all day. You are refilling a reservoir, not supplying a tap.

It does not need to be big. Its size follows the charger, not the house.

Size it to the charger, not the loads

Work out what your inverter/charger can actually draw on AC input. That figure is on its data sheet, usually as a maximum input current.

A charger accepting 40 amps at 120 volts asks for roughly 4,800 watts. That is a continuous demand, so it wants a generator rated for that continuously, not one that reaches it as a surge.

Then stop. Do not add your household loads on top, because they are being served by the battery. Do not round up “for headroom” either. A generator running at a fraction of its rating burns fuel to make heat and noise, and wet stacking is a real failure mode on diesels run lightly for years.

Most off-grid systems are well served by something in the 3,000 to 7,000 watt range. Use the appliance power budget and system sizing calculators to establish the bank first, because the charger follows the bank and the generator follows the charger.

The problem nobody warns you about

Here is the failure that sends people to forums: the generator runs perfectly, and the inverter/charger refuses to connect to it.

Modern inverter/chargers inspect incoming power before they accept it. They check voltage, frequency, and waveform quality, and they reject anything outside their window rather than pass bad power through to your system. A cheap open-frame generator without automatic voltage regulation can drift outside that window under changing load, and a generator with high harmonic distortion can be refused even when the voltage looks correct on a meter.

Nothing is broken. The generator will happily run a drill. The two devices simply disagree about what acceptable power is, and the inverter/charger gets the final say.

This is the single most important thing to check before buying. Two specifications decide it:

Automatic voltage regulation. Holds output steady as load changes. Treat its absence as disqualifying.
Total harmonic distortion, or THD, which measures how far the waveform departs from a clean sine wave. Lower is better, and inverter generators typically achieve under 3%.

The awkward part is that THD is inconsistently published. Inverter generators usually state it because it is a selling point. Conventional open-frame models often do not, and silence is not a good sign.

An inverter generator is the safer choice for this reason, alongside being quieter and more economical at partial load. It costs more per watt, and for a machine that runs a few dozen hours a year that is usually money well spent.

Fuel: propane, in most cases

Dual-fuel generators run on gasoline or propane. Off-grid, propane is usually the better answer, and the reason is storage rather than combustion.

Gasoline degrades. It can begin going stale within about thirty days, faster with ethanol blends, and what it leaves behind gums up a carburetor. A generator that runs thirty hours a year spends the other 8,730 sitting still, which is exactly the duty cycle that ruins gasoline machines.

Propane does not degrade. It stores indefinitely, burns cleaner, and leaves fewer deposits. Most off-grid properties already keep a propane tank for cooking or heating, so the fuel and the delivery arrangement exist already.

The trade is power. The same engine produces less on propane than on gasoline. Published derate figures vary widely enough that a rule of thumb is not much use, so take the number from the manufacturer’s own spec sheet, which lists separate running watts for each fuel. Size against the propane figure if propane is what you will actually burn.

Tri-fuel and natural gas

A tri-fuel generator, or a conversion kit that adds the capability, runs on gasoline, propane, and natural gas.

Off-grid, the third fuel is usually unusable. Natural gas arrives through a utility connection, and a property without utility electricity rarely has utility gas. If you are choosing between dual fuel and tri fuel for an off-grid site, the extra capability is likely to sit idle, and propane is the fuel that matters.

Where natural gas genuinely helps is a grid-connected house wanting backup, because an unlimited fuel supply with nothing to refill is a real advantage during a long outage. That is a different setup with different rules, and it belongs to the backup section rather than to off-grid design.

If you do convert, three things are worth knowing. Natural gas derates output further than propane does. The supply line has to be sized for the flow the generator wants, and an undersized line starves the engine under load. And connecting a gas appliance is licensed work in many jurisdictions, in the same way the electrical side is.

Carbon monoxide

A generator produces carbon monoxide in quantities that kill quickly, and it is odorless.

Never run one indoors, in a garage, in a shed, or near an opening into living space, including with a door open. Distance and open air are the only real protections. Put a carbon monoxide alarm in any building near where a generator runs, and treat that alarm as maintained equipment rather than something installed once.

This is the hazard on this page most likely to actually hurt someone, and it is not a small margin.

What to check before buying

Automatic voltage regulation, present.
THD published, and low. Absence of a published figure is itself information.
Continuous rating, not peak, at or above your charger’s AC input demand.
Running watts on propane specifically, if propane is the plan.
Whether your inverter/charger has a documented generator input and any stated compatibility requirements.

Naming specific models is worth doing carefully rather than quickly, because THD is the deciding specification and it is exactly the one manufacturers are least consistent about publishing.

Common questions

Do I need a generator for an off-grid system?: At full-time scale, almost certainly. The last two days of autonomy are the most expensive and least used part of a battery bank, and no amount of storage covers a week of snow on the array. A generator handles the rare long gap for a fraction of what the equivalent battery costs.
What size generator do I need off-grid?: Size it to what your inverter/charger can draw, not to your household loads. If the charger accepts 40 amps at 120 volts, that is about 4,800 watts of demand, and the generator has to supply that continuously rather than as a surge. Oversizing wastes fuel because a lightly loaded engine is inefficient.
Why will my inverter/charger not accept the generator?: Almost always power quality. Inverter/chargers check incoming voltage, frequency and waveform before connecting, and a generator without automatic voltage regulation can drift outside those limits. The generator runs fine on its own and still gets refused, which makes this look like a fault when it is a compatibility problem.
Propane or gasoline for an off-grid generator?: Propane, in most cases. It stores indefinitely, where gasoline can start degrading within about thirty days and gums a carburetor when a machine sits unused. Off-grid properties usually have a propane tank already. The trade is a lower power rating on propane than on gasoline from the same engine.
Is a tri-fuel conversion worth it off-grid?: Usually not, because the third fuel is natural gas and that requires a utility gas line. A property without utility electricity rarely has utility gas. If you do have gas service and want backup for a grid-connected house, that is a different setup and a different set of rules.