Which generator fuel keeps, and for how long

The usual fuel comparison is about running cost. Pennies per kilowatt hour, efficiency at half load, which machine is cheapest to feed.

For a generator that might sit untouched for two years and then be needed at three in the morning, that is the second question. The first is whether the fuel in the can is still fuel.

Shelf life, plainly

Petrol is the worst of the three by a distance. Three to six months in a sealed container is the usual expectation before it degrades noticeably, and a stabiliser added to fresh fuel takes that toward twelve months or more depending on the product.

Diesel does better, roughly a year, with its own complications around microbial growth and water in the tank over longer periods.

Propane does not degrade at all. It is a simple, stable molecule in a sealed vessel, and a cylinder stored dry and upright holds usable fuel for decades. The limiting part is the container, which can rust or develop a valve fault, which is why cylinders carry recertification dates.

Why petrol goes off

Two mechanisms, and both are worse in a vented can in a warm garage.

The light fractions evaporate, which changes how readily the fuel ignites. And the remainder oxidises, forming gums and varnish that coat carburettor passages. The result is a machine that cranks and will not start, or starts and runs badly, which is discovered precisely when it is needed.

Ethanol blends add a third problem. Ethanol is hygroscopic, so it pulls moisture out of the air, and past a point the water separates out and sits at the bottom of the tank. That is phase separation, and the engine will not run on what settles there.

What to do if petrol is your fuel

Treat it as a rotating stock rather than a reserve. Buy stabilised fuel or add stabiliser when it is fresh, label the can with the date, and pour it into the car every six months, replacing it with new.

Ethanol-free petrol, where it is available, stores considerably better and is worth the premium for a machine that sits idle. And run the carburettor dry before storing the generator, or use the fuel shutoff and let it stall, so there is no fuel left sitting in the bowl turning to varnish.

Matching fuel to how you will use it

The choice falls out of the usage pattern more than the price list.

For a standby unit on natural gas, the question does not arise, which is a large part of their appeal. The supply is piped, it does not age, and run time is limited only by the gas network.

For a portable used a few times a year, propane is usually the better default. Dual fuel machines run on either, which means petrol when it is to hand and propane as the reserve that will still work in three years. Consumption on propane is a little higher for the same output, and the generator fuel calculator will show what that difference costs over a realistic outage, which is normally less than people expect.

For a machine that runs regularly, a work site generator or one used for a trailer, petrol is fine, because the fuel never sits still long enough to spoil.

Storage quantity is a safety question

Fuel storage is governed by local rules, and the limits are there for good reason. Vapour is heavier than air, it collects at floor level, and a water heater or a furnace with a standing pilot in the same space is a plausible ignition source.

Approved containers, modest quantities, well ventilated space, away from living areas and away from anything with a flame. Propane cylinders in particular belong outdoors or in a ventilated outbuilding rather than in a basement, where a leak has nowhere to go.

The maintenance that matters more than the fuel

A generator that fails to start after storage usually fails for one of three reasons, and only one of them is the fuel.

Stale petrol in the carburettor is the first. A flat starting battery is the second, which is why standby units exercise weekly and why the battery is a consumable rather than a fitting. Old oil that has sat in the sump absorbing moisture is the third.

Running the machine under some load for half an hour twice a year addresses all three at once, and it is the single habit that separates a generator that works in an outage from one that is a heavy object in the garage.