Natural gas vs. propane generator: which fuel?

A standby generator runs on a piped fuel you don’t have to refuel by hand, which is the whole point. For most homes that means natural gas or propane (LP). The choice changes where you can put the unit, what it costs to run, and how it behaves in a long outage.

The two fuels

Natural gas comes from a utility line, so you never run out as long as the line is up. It’s metered and billed monthly, and most homes that have it already use it for heat or a stove. A generator on natural gas runs indefinitely without you touching it.

Propane is stored in a tank you own or lease, and you have it delivered. The tank sits above or below ground near the generator. A propane generator runs as long as the tank has fuel, which means a long outage tests your tank size, not the utility line.

Both are piped to the generator and ignited automatically by an automatic transfer switch. Neither fuel is something you carry a can of, which is what makes them suitable for standby duty in the first place.

Supply and storage

The practical difference is uptime risk:

• Natural gas supply is continuous and rarely interrupted, but it can be in an earthquake or a major pipeline event. It also depends on the utility’s pressure holding up during a widespread outage, which it usually does.
• Propane is finite to the tank. A 500-gallon tank runs a 22 kW generator for roughly a week of continuous duty, depending on load. A bigger tank or a topped-off tank is your insurance against a long outage, and you schedule the delivery yourself.

If you don’t have a natural gas line, propane is the standby option. If you do have gas, you choose between the convenience of never running out and the resilience of stored fuel.

Output and derating

A generator’s rated wattage is usually stated on natural gas. On propane the same unit may produce slightly less, often a few percent, because propane carries a bit less energy per volume at the metering the generator expects. Manufacturers publish both ratings. It’s rarely the deciding factor, but it’s why two “22 kW” listings can show different numbers depending on fuel. Check the fuel spec on each model page, not just the headline kW.

Running cost

Natural gas is usually cheaper per hour of run time where you have a line, because you pay by the therm and there’s no tank or delivery. Propane is more per hour but you buy it ahead, so the cost is known and not on a monthly bill. During an outage you run the generator for days, not weeks, so the difference is real but small next to the installed cost. See what a whole-house generator costs installed for the bigger picture.

Dual-fuel hedges the choice

Most residential standby units today are dual-fuel, set up by the installer for natural gas or propane, sometimes with a conversion kit. That means you don’t have to commit forever. If your area gets natural gas later, or you switch from a tank to a line, the generator can usually follow. Confirm the fuel spec on the model before you buy.

Pick the fuel after the size

Fuel doesn’t change what size you need. Size the generator first, then choose natural gas if you have the line and want unattended uptime, or propane if you don’t have a line or want the resilience of stored fuel. Compare our standby catalog filtered by fuel once you know both.