Generator Fuel Types: Natural Gas vs Propane vs Gas vs Diesel

The right generator fuel depends less on price than on what you can actually get when the power is out. If a natural-gas line already serves your home, piped natural gas is almost always the best choice for a standby unit: effectively unlimited runtime during an outage, the lowest operating cost, and no tank to store or refill. Off the gas main, propane is the standard for standby generators — clean-burning, stores for years without going stale, and good for days of runtime depending on tank size. Gasoline rules the portable category because it's available everywhere and the engines are cheap, while diesel shows up mainly on large standby and commercial units where rugged engines and high energy density matter more than noise or emissions. This guide compares all four on cost, outage availability, storage and shelf life, runtime, maintenance, and emissions — and which fuel fits portable versus standby use.

Generator fuels at a glance

Natural gas — the best choice when a line is available

If your home is already served by a natural-gas line — the same supply that feeds a furnace, water heater, or range — natural gas is usually the best fuel for a standby generator. Three advantages stand out: effectively unlimited supply during an outage, the lowest operating cost of any fuel, and nothing to store. There's no tank to install, no level to monitor, and no delivery to schedule in the middle of a storm.

The supply advantage is the big one. Natural gas arrives through underground pipelines whose infrastructure is largely independent of the electric grid, so the gas keeps flowing even when the power is out. A natural-gas standby unit can run for days without anyone touching it — the limiting factor becomes engine maintenance intervals, not fuel.

The tradeoffs are real but narrow. Natural gas is standby only: portable generators can't tap a fixed gas line, so this fuel is off the table for grab-and-go use. Output is also marginally lower than the same engine on gasoline, because natural gas carries slightly less energy per unit volume — generator makers publish separate ratings for each fuel. And of course you need a gas line at the property in the first place; extending a main to a home that isn't served can be expensive, so natural gas mostly makes sense where the connection already exists.

Choose natural gas when:

• Your home already has a natural-gas connection
• You want a standby generator that runs indefinitely without refueling
• You'd rather not manage a fuel tank at all
• Lowest possible operating cost is a priority

Propane (LP) — the standard off the gas main

Propane is the dominant fuel for standby generators anywhere a natural-gas line doesn't reach. It's delivered by truck and stored in an on-site tank, or — for portable units — bought in exchange cylinders. Its defining strength is shelf life: propane is stored under pressure as a liquid and doesn't go stale, so a tank filled today is still usable years from now. That makes it ideal for a generator that may sit idle for long stretches between outages.

Tank sizing and runtime

Standby propane generators run off a stationary tank, typically aboveground, sized to the runtime you want:

• 120-gallon (about 420 lb) tank — a common minimum for a small standby unit; useful for shorter outages.
• 250-gallon tank — a typical residential standby choice, balancing footprint and several days of runtime.
• 500-gallon tank — a frequent pick where longer outages are common or the home draws heavily; days of runtime with margin.
• 1,000-gallon tank — for large homes, rural properties, or regions prone to multi-day outages.

Actual runtime depends on generator size and load. As a rough planning figure, a mid-size home standby unit running at half load burns roughly 2–3 gallons of propane per hour, so a 250-gallon tank (filled to the usual ~80% safe capacity, about 200 usable gallons) gives on the order of several days of continuous power. Heavier loads burn faster; light loads stretch much further.

Storage and safety

Propane is heavier than air, so a leak settles low rather than dispersing. Codes reflect that — tanks must sit a minimum distance from building openings and ignition sources, rest on a stable pad, and be inspected periodically. A practical convenience is an automatic-delivery contract with a tank-level monitor: the supplier refills before you run low, so you don't drain the tank mid-storm. One cold-weather caveat: in extreme cold, propane vapor pressure drops, which can limit output from smaller tanks — larger tanks and proper sizing mitigate this.

Choose propane when:

• No natural-gas line serves the property
• You want a standby generator with clean-burning, never-stale fuel
• The generator may sit unused for months between outages
• You're considering a dual-fuel portable that needs long fuel storage

Gasoline — the portable default

Gasoline is the default fuel for portable generators. The reasons are practical: it's sold at every gas station, the engines are inexpensive, and you can carry fuel in cans. For short, occasional outages, a gasoline portable is the cheapest way to keep a refrigerator, a few lights, and a sump pump running.

The weaknesses are shelf life and refueling. Untreated gasoline begins to degrade in roughly 3–6 months and can gum up a carburetor well before a year — a serious problem for a generator that mostly sits unused. A fuel stabilizer plus a sealed, cool container stretches usable life to about 1–2 years, and running the carburetor dry before storage helps. The other catch is a long outage: gasoline burns through cans quickly, and refueling a hot engine means shutting down, letting it cool, and hauling more fuel — often when stations are closed or out of power themselves.

Many modern portables are now dual-fuel (gasoline plus propane), which sidesteps the worst of gasoline's problems: run on propane for storage and quiet outages, fall back to gasoline when that's what's on hand.

Choose gasoline when:

• You want an inexpensive portable for short or rare outages
• Fuel availability everywhere matters more than long storage
• You'll use a stabilizer or run a dual-fuel unit on propane between events

Diesel — runtime and durability at scale

Diesel is uncommon on small residential portables but common on large standby units, farms, and commercial installations. It's the fuel of choice once you get into bigger generators where rugged engines and long runtime per gallon matter more than noise.

Strengths

• Energy density and runtime — diesel packs more energy per gallon than gasoline, so a given tank runs longer at the same load.
• Engine durability — diesel engines are built for long service life and heavy, sustained use.
• Reasonable storage — diesel stores longer than gasoline, roughly 6–12 months untreated and longer with biocide and stabilizer added.
• Fuel availability — diesel is widely available by delivery, even in rural areas.

Tradeoffs

• Noise and emissions — diesel engines run louder and produce more NOx and particulates than gas or propane, which is a concern in dense residential settings.
• Cold-weather gelling — straight diesel can gel in cold temperatures; winter blends and anti-gel additives are needed in cold climates.
• Maintenance — diesels need fuel filters changed on schedule, periodic fuel polishing or treatment to prevent microbial growth (diesel attracts moisture), and tank condensation managed.
• Cost — diesel generators and their engines generally cost more upfront than equivalent gas or propane units.

For most standard homes, propane or natural gas is the better standby choice. Diesel earns its place where loads are large, runtime is critical, and the engine will see heavy, sustained use.

Storage and shelf life compared

Shelf life is where the fuels diverge most, and it often decides which one suits how you'll actually use the generator:

• Natural gas — nothing to store; piped in fresh on demand.
• Propane — stores for years in a sealed tank without degrading.
• Diesel — about 6–12 months untreated; longer with stabilizer and biocide.
• Gasoline — about 3–6 months untreated; 1–2 years with stabilizer.

For a generator that sits idle between rare outages, this chart alone argues for propane (or natural gas) over gasoline. For a unit you'll run and refill regularly, gasoline's short shelf life matters far less.

Which fuel fits portable vs standby?

The portable-versus-standby split lines up cleanly with the fuels:

• Portable → gasoline or dual-fuel (gas/propane). You carry the fuel, the engine is cheap, and a manual transfer setup ties it to your panel. Best for short, occasional outages.
• Standby → natural gas if a line is available, otherwise propane. Permanently installed, automatic startup, runs for days. Best for whole-home backup and longer outages.
• Large standby / commercial → diesel, where runtime, durability, and energy density outweigh noise and emissions.

To weigh backup power against your normal utility bill, see the related electricity cost calculator below, and check typical rates on our electricity rates page. For more on the rest of the decision, read our companion guides on how to size a home generator and portable vs standby generators, or browse all home energy guides and the full guide library.

The bottom line

Pick the fuel by availability first, then cost. Natural gas wins when a line reaches the property — cheapest to run, unlimited during outages, nothing to store. Off the main, propane is the standby standard: clean, never stale, sized to the runtime you need. Gasoline owns the portable category for short outages but demands a stabilizer and frequent refueling. Diesel earns its keep on large and commercial units where runtime and durability rule. Match the fuel to how long your outages last and whether you want a portable or a standby — not to the headline price per gallon.