How Long Does Gas Last in a Generator Before it Goes Bad?

The storm track shifts your way, you pull the generator out of the garage, and the gas in the tank is whatever was left in it last October. Now you have a decision to make with the wind picking up and the hardware store already out of gas cans.

How long does gas last in a generator? There isn't one expiration date that applies to every tank of gasoline. Fuel age, storage conditions, ethanol content, temperature, and whether the gasoline was properly treated all matter. What we can say is that untreated gasoline can begin degrading surprisingly quickly—and a generator is one place where you don't want to discover a fuel problem when you finally need the machine to run.

How long does gas last in a generator, treated and untreated?

Untreated gasoline can begin degrading in a surprisingly short period of time. Briggs & Stratton says gasoline can begin to degrade in as little as 30 days, potentially contributing to gum and varnish formation, fuel-system clogging, and starting problems.

Generac also treats 30 days as an important threshold for portable-generator fuel storage. Its current guidance recommends draining the fuel before storing a portable generator for more than 30 days, while its troubleshooting guidance warns that gasoline stored in the generator for more than 30 days without stabilizer may contribute to carburetor problems.

That doesn't mean every gallon of untreated gasoline becomes unusable after 30 days. Fuel degradation is a process, not an expiration date. How quickly it becomes a problem depends on the fuel itself and the conditions under which it is stored.

Heat, exposure to air, moisture, and repeated temperature changes can all work against stored gasoline. Ethanol-blended gasoline brings an additional concern because ethanol has a strong affinity for water. If enough water gets into the fuel, the ethanol-water portion can eventually separate from the gasoline in a process called phase separation.

The generator is the durable part of this equation. Bell Performance has been working with fuel chemistry since 1909, and stored fuel is one of the first things worth looking at when a generator that has been sitting suddenly becomes hard to start. If you want the full picture across gasoline and diesel units, Bell's generator fuel storage guide covers both.

What happens to gasoline sitting in a generator tank?

Several things can happen to gasoline while it sits. The fuel gradually oxidizes, producing gums and other degradation products that can leave deposits in the fuel system and carburetor. At the same time, changes in the fuel's more volatile components can make old gasoline less suitable for easy starting and good combustion.

Ethanol-blended gasoline adds another consideration. Ethanol has an affinity for water and allows the gasoline blend to hold more dissolved water than ethanol-free gasoline can. But there is a limit. If enough water enters the fuel—or conditions change enough to reduce how much water the fuel can hold—the ethanol and water can separate from the gasoline and settle into a distinct lower layer. That's phase separation.

Phase-separated fuel is particularly troublesome because the lower ethanol-and-water-rich layer can be corrosive and may be drawn into the fuel system instead of normal gasoline.

And generator carburetors don't give you much margin for error. Their small jets and passages can be restricted by deposits from deteriorated fuel, while water and phase-separated fuel can create additional corrosion and performance problems. None of this may become obvious until the day you actually need the generator.

How do you know if the gas in your generator has gone bad?

You can sometimes spot obvious problems with old generator fuel by looking at it and smelling it, but appearance alone can't tell you that old gasoline is still good.

Draw a small sample into a clear glass container and compare it, if possible, with fresh gasoline. Fuel that has become noticeably darker, has visible sediment or contamination, or has developed an unusual sour or varnish-like odor has clearly changed during storage.

Two distinct liquid layers are an even clearer warning sign with ethanol-blended gasoline. That can indicate phase separation, where an ethanol-and-water-rich layer has separated from the gasoline and settled to the bottom. Fuel in that condition should not be put back into service.

What the jar test cannot do is prove that gasoline is still within specification simply because it looks clear and remains in one layer. Gasoline can lose important volatility and performance characteristics without producing an obvious visual change.

The generator can provide clues too. Hard starting, surging, stalling, or an engine that will only run with the choke partly closed can all be consistent with a restricted carburetor or fuel-delivery problem. Old gasoline is one possible cause and should be considered when the generator has been sitting for an extended period.

Can you use last year's gas in your generator?

Maybe—but age alone doesn't tell you whether last year's gasoline is still suitable for use.

If the fuel was treated when it was fresh, stored properly, and remains within the storage period recommended by the stabilizer and equipment manufacturer, it may still be usable. Untreated gasoline that has been sitting for many months deserves considerably more caution.

Look first for obvious warning signs: phase separation, water, sediment, unusual discoloration, or a strong sour or varnish-like odor. If any of those are present, don't try to rescue the fuel by adding fresh gasoline or pouring in an additive.

Adding fresh gasoline can dilute moderately aged fuel, but it cannot reverse oxidation, restore fuel that has severely deteriorated, or recombine gasoline that has already phase-separated. And a simple visual inspection cannot tell you whether old gasoline still meets all of the volatility and performance characteristics it had when it was fresh.

For an emergency generator—equipment you are depending on to work when the power is out—the safest approach with questionable year-old fuel is to remove it and start with fresh gasoline rather than gamble on whether an old batch will still perform.

If the fuel needs to be removed, handle and dispose of it according to local requirements. Bell walks through that judgment call in what to do with old gas and can I use last year's gas in my generator.

What is the best way to keep generator gas fresh?

The best time to protect stored generator fuel is when the gasoline is still fresh. Add the appropriate fuel treatment when you buy the gasoline, store the fuel properly, and make sure treated fuel reaches the generator's fuel system rather than treating only the gasoline sitting in the tank.

Ethanol Defense is built for this job. It combines professional-strength, alcohol-free water control with detergency and protection designed for the problems associated with ethanol-blended gasoline. Its water-control chemistry helps manage moisture before it can contribute to phase-separation problems, while its detergent package helps control deposits in the fuel system.

One important point is easy to overlook: treating gasoline in the tank doesn't help fuel already sitting untreated in the carburetor. After adding treated gasoline, follow the generator manufacturer's instructions for circulating treated fuel through the fuel system.

For longer-term storage, always check the owner's manual for the particular generator. Manufacturers don't all recommend exactly the same procedure. Some recommend storing with properly stabilized fuel, while others recommend draining the fuel system or running the generator dry before extended storage.

How should you store gas cans for a portable generator?

Store generator gasoline in approved, tightly closed fuel containers and keep them in a cool, well-ventilated location away from living spaces, ignition sources, and anything that can produce a flame or spark. Follow the container manufacturer's instructions and applicable local fire and storage requirements.

Minimizing unnecessary air space can help reduce the fuel's exposure to air and moisture, but never overfill a gasoline container. Leave the expansion space required by the container manufacturer.

Keeping your reserve fuel separate from the generator's own tank also makes it easier to rotate your supply. How much fuel to keep on hand depends on your generator's fuel consumption at the load you expect to run and how many hours per day you expect to operate it.

The best strategy is rotation rather than trying to store gasoline indefinitely. Periodically use your oldest properly stored fuel in an appropriate gasoline-powered vehicle or piece of equipment and replace it with fresh, treated gasoline. That keeps your emergency supply from quietly becoming several years old before you discover it.

Still running last season's gas?

If the gas in your generator is left over from last season, don't wait for an outage to find out what condition it's in. Inspect the fuel now, and if there is any real question about its condition, replace it with fresh gasoline.

For fresh gasoline going into storage, Ethanol Defense uses professional-strength, alcohol-free water-control chemistry to help manage the moisture problems associated with ethanol-blended gasoline, along with a detergent package that helps keep the fuel system clean.

The best time to treat generator fuel is before storage problems develop—not after badly degraded fuel has already caused them. Ethanol Defense treats at 1 oz per 10 gallons, so a 16 oz bottle treats up to 160 gallons of gasoline.

See Ethanol Defense treat rates and order