Storing Gasoline for a Generator

The most common generator failure isn't mechanical. It's a generator that won't start because the fuel in it has turned to varnish over eleven months of sitting.

It's also the most preventable failure on the list.

Fuel storage life

Stale gasoline is the top reason a portable generator will not start when the outage arrives. Ethanol attracts water and the light fractions evaporate, leaving varnish in the carburettor.

Either treat the fuel and rotate it, or choose propane — which is exactly the argument for a dual fuel unit.

Why does gasoline go bad?

Oxidation. Gasoline reacts with oxygen over time, forming gums and varnish that coat carburetor jets and passages. Small engines have very fine passages, so they're affected sooner than a car would be.

Ethanol absorbs water. Most US pump gasoline contains ethanol, which is hygroscopic — it pulls moisture out of the air. Absorbed water can separate out and settle, causing corrosion and poor running.

Volatiles evaporate. The light components that make an engine start easily are the first to go, so old fuel is specifically harder to start on, even when it will run.

Untreated pump gasoline typically starts degrading within a few months. In a generator that sits unused for a year, that's the whole story.

How long does fuel stabilizer extend gasoline?

Fuel stabilizers slow oxidation and inhibit gum formation. Storage life with stabilizer is commonly cited as up to a year or more, depending on product and conditions.

Two rules:

Add it to fresh fuel. Stabilizer prevents degradation; it doesn't reverse it. Adding it to fuel that's already stale accomplishes nothing.

Run the engine after adding it. Treated fuel needs to reach the carburetor, not just sit in the tank. A few minutes of running circulates it through the fuel system.

Is ethanol-free gas better for a generator?

Where you can get it — marinas, some stations, and small pre-mixed cans sold for small engines — ethanol-free gasoline stores considerably better and avoids the water absorption problem entirely.

Pre-mixed canned fuel is expensive per gallon but has a very long shelf life, and it's a reasonable choice for a small reserve you want to be certain about.

How do you store gasoline safely?

Approved containers only. Fuel cans rated for gasoline, properly sealed. Not milk jugs, not water containers, not anything unrated.

Never in the house. Not in a basement, not in a utility room, not in an attached garage if you can avoid it. Gasoline vapor is heavier than air, travels along floors, and finds ignition sources — a water heater pilot, a furnace, a light switch.

A detached, ventilated shed or outbuilding is the right place.

Away from ignition sources. Any appliance with a pilot light or a spark.

Out of sunlight, in a cool location. Heat accelerates degradation and raises vapor pressure.

Check local fire code limits on stored quantity. They vary by jurisdiction, and they exist for good reason.

Label with the fill date. You will not remember.

Rotation

The habit that actually prevents no-start failures:

1. Store what you'll realistically use, not the maximum you're allowed
2. Add stabilizer when filling the containers
3. Rotate every 6–12 months — pour the stored fuel into your car and refill the cans fresh
4. Note the date on each can

Rotating into a car is the easy part: a vehicle burns the fuel long before it matters, and modern cars tolerate slightly aged fuel fine.

What to do with the generator between uses

Two schools, both valid:

Run it dry. Shut off the fuel valve and let the engine run until it stops. No fuel remains in the carburetor to varnish. Best for long storage.

Store it full with stabilizer. A full tank leaves less air space for condensation, and stabilized fuel keeps. Requires actually running it periodically so treated fuel circulates.

What not to do: leave untreated fuel sitting in the carburetor for a year. That's the failure mode.

Either way, run the generator under load periodically — commonly recommended monthly for 15–30 minutes. It circulates fuel and oil, keeps seals conditioned, and reveals problems while you have time to fix them. See generator maintenance schedule and generator exercise mode.

Refuelling safety

Never refuel a running or hot generator. Spilled gasoline on a hot exhaust or muffler ignites. Shut it down and let it cool — genuinely cool, not "a few minutes".

This is a real and recurring cause of generator fires during outages, when people are tired and want to keep the power on.

Refuel outdoors, away from ignition sources, with a funnel or spout to avoid spills.

Don't overfill. Fuel expands with temperature, and an overfilled tank pushes fuel out.

What do you do with stale generator fuel?

Symptoms: the engine won't start, starts and stalls, runs rough, or surges.

• Drain the old fuel and dispose of it properly — most areas have household hazardous waste collection. Never pour it on the ground or down a drain.
• Clean the carburetor, which is usually where the varnish is. This is the most common generator repair and it's often straightforward.
• Refill with fresh, stabilized fuel.

See generator won't start: troubleshooting.

The propane alternative

Worth restating: propane stores indefinitely. No stabilizer, no degradation, no gumming, no rotation habit.

For a generator that sits unused between outages — which describes almost all of them — that's the strongest argument for a dual-fuel unit. See dual fuel and tri-fuel generators.

Where to go next

• Generator won't start: troubleshooting
• Generator maintenance schedule
• Generator fuel types
• Size it properly: generator sizing calculator

More in our generator and backup power guides.