What Do Backup Generators Run On? | Fuel By Generator Type

Backup generators run on natural gas, liquid propane, diesel, or gasoline, with home standby units using gas or propane.

Peeking behind the house at a new generator, most people wonder what fuels it. The answer is simpler than it looks, and it comes down to one thing: the type of generator you need. Natural gas and liquid propane power most American homes, while diesel and natural gas dominate critical commercial facilities. The choice affects everything from installation cost to what happens during an outage, so it’s worth getting right before you buy.

Home Standby Generators: Natural Gas Or Liquid Propane

Home standby generators are the permanent units that sit outside on a concrete pad, like a central air conditioner. They connect to your home’s fuel supply and start automatically when the power goes out.

Generac, the largest home standby manufacturer, states that these units are designed to run on either natural gas or liquid propane. The key is that a given model is configured for one fuel source or the other—you don’t get to flip between them freely. A natural-gas unit ties into your home’s existing gas line, giving you unlimited runtime so long as the gas keeps flowing. A propane unit draws from an on-site tank; it gives you independence from the utility grid but requires keeping the tank filled.

Most homeowners choose natural gas because it requires no refueling and ties into the fuel they already use to heat their homes and cook their food. But if your home isn’t on a gas line, a large propane tank is the common workaround, offering the same automatic operation without needing utility gas.

Portable And Commercial Backup Generators: Different Fuels

Portable backup generators—the wheeled units you roll out during an outage—run primarily on gasoline or diesel. Gasoline is the most common for these units because it’s readily available at any station, but it has one major drawback: gasoline degrades over time. If you store it for months between outages, you’ll need to treat it with stabilizer or rotate your supply, and a generator that sat with stale gas often won’t start when you need it most.

Diesel is more common in larger, commercial-grade portable units. Diesel fuel also stores better than gasoline, and diesel engines are built heavier, making them a favorite for contractors and farms where the generator gets used regularly, not just in emergencies.

At the commercial and critical-facility level, the picture shifts again. According to the National Renewable Energy Laboratory (NREL), roughly 95% of backup generators in commercial buildings and critical facilities run on either diesel or natural gas.

Fuel Type Comparison

Here’s the quick reference on what runs what:

Safety Rules For Any Generator Fuel

Whatever fuel your generator uses, four safety rules apply without exception.

Run the generator outdoors only. Generators produce carbon monoxide, a colorless, odorless gas that kills quickly in enclosed spaces. Never run one inside a garage, basement, or near a window or door where exhaust can drift into the house.

Never connect a generator directly to your home’s wiring by plugging it into a wall outlet. That backfeeds power into the grid and can electrocute utility workers. A transfer switch—installed by a licensed electrician—is the only safe way to connect a portable generator to your home’s circuits. A properly installed transfer switch also keeps your generator from overloading when the power comes back.

Never refuel a running generator. Turn it off and let it cool completely first; gasoline spilled on a hot engine can ignite. Store fuel only in approved containers, outside living areas, and away from any fuel-burning appliance.

Use heavy-duty, outdoor-rated extension cords, and if you’re still comparing models for portability and home use, our portable backup generator roundup covers tested options for that route.