Propane vs. gasoline for a generator: power loss, runtime and storage

Dual-fuel portable generators run on gasoline or propane at the flip of a switch, and they've become the default choice for home backup. Picking the right fuel for an outage comes down to three trade-offs: propane makes a little less power, burns faster by volume, and stores far better. This guide puts numbers on each, using manufacturer spec sheets and federal energy data, so you can plan fuel for the generator you have.

Propane carries less energy per gallon

The U.S. Energy Information Administration lists the heat content of motor gasoline at about 120,214 Btu per gallon and propane at about 91,452 Btu per gallon. A gallon of propane holds roughly 76 percent of the energy of a gallon of gasoline.

That matters for runtime by volume, but people rarely buy propane by the gallon for a portable generator. They buy it in 20-pound cylinders, the kind used for grills. Propane weighs a little over 4 pounds per gallon as a liquid, so a full 20-pound cylinder holds roughly 4.7 gallons, or about 430,000 Btu. For comparison, 5 gallons of gasoline holds about 600,000 Btu. In practice, a 20-pound tank gives you somewhat less runtime than a 5-gallon gas can, and a lot less than the full gas tank on most mid-size generators.

Dual-fuel generators make less power on propane

Every dual-fuel spec sheet lists two sets of ratings, and the propane numbers are lower. Here are three common 7,500 to 9,500-watt models, from the manufacturers' own pages:

Model	Running watts, gasoline	Running watts, propane	Drop
Champion 201568	8,000	7,200	10%
Generac GP7500E dual fuel	7,500	6,800	9%
Westinghouse WGen9500DFc	9,500	8,500	11%

Starting (surge) watts drop by a similar share. Champion's model goes from 10,000 starting watts on gasoline to 9,000 on propane.

A 10 percent drop is easy to miss until it matters. If you sized your generator to run a well pump, refrigerator and furnace blower with little headroom on gasoline, the same load may trip the breaker on propane, especially when the well pump starts. When you use our generator size calculator, compare your total against the propane rating if propane is the fuel you plan to rely on. Our guide to starting vs. running watts explains why the surge figure is usually the limit.

Cold weather can shrink propane output further. Propane must vaporize in the cylinder before it reaches the engine, and a small tank feeding a large engine in freezing weather may not vaporize fast enough. That shows up as surging or an inability to carry a heavy load. Keeping the tank out of snow and using a larger tank helps.

Runtime: what the spec sheets say

Manufacturers list runtime at 50 percent load (sometimes 25 percent). For the models above:

• Champion 201568: 10 hours at half load on its 7.7-gallon gasoline tank, 5 hours at half load on a 20-pound propane tank.
• Westinghouse WGen9500DFc: 8 hours at half load on 6.6 gallons of gasoline, 7 hours at half load on a 20-pound tank.
• Generac GP7500E: up to 10 hours at half load on its 7.9-gallon gasoline tank.

Two things stand out. First, a 20-pound tank lasts roughly 5 to 7 hours at half load on a generator this size, so a day of running through an outage takes about three to five cylinders. Second, the gasoline figures come from a full tank, which holds more energy than one cylinder. Gallon for gallon, the fuels aren't as far apart as the runtimes suggest.

Real loads aren't steady. A refrigerator cycles, a well pump runs in bursts, and you may only run the generator a few hours at a time. Our fuel runtime estimator and guide to how long a generator can run help you turn your own load into hours.

Planning propane for an outage

Work backward from the number of hours you want covered.

1. Find your generator's propane runtime at the load closest to yours. If the spec sheet only lists 50 percent load and your load is lighter, you'll get more hours, though not proportionally more.
2. Divide hours needed by hours per tank. Three days of running 12 hours a day is 36 hours. At about 5 hours per 20-pound tank, that's seven to eight cylinders.
3. Consider a bigger tank. Many dual-fuel generators can run from 30-, 40- or 100-pound cylinders with the right hose and regulator, which avoids swapping tanks at 2 a.m. Check the generator manual for approved setups.
4. Store cylinders outside. Propane cylinders belong outdoors, upright, away from the house, and never in a basement, garage or living space.

Planning gasoline for an outage

Gasoline's problem isn't energy. It's shelf life and storage. Untreated gasoline starts to degrade within months, and ethanol blends absorb water. Stale fuel is one of the most common reasons a generator won't start after sitting. If gasoline is your outage fuel:

• store it in approved containers, outside the living space and away from ignition sources;
• add a fuel stabilizer when you fill the cans, following the product's directions;
• rotate stored fuel into your car every few months and refill;
• run the generator dry or use stabilized fuel before long storage, as the manual recommends.

Our guide to storing generator fuel covers containers, stabilizers and local limits on how much you can keep.

Which fuel should you use?

Propane is the better choice when:

• the generator sits for months between outages (propane doesn't go stale, and the carburetor doesn't gum up);
• you want to stock fuel ahead of hurricane or winter-storm season;
• you have a grill tank or larger cylinders already;
• gas stations may be closed or without power during a long outage.

Gasoline is the better choice when:

• you need every watt the generator can make;
• temperatures are well below freezing and you only have small cylinders;
• you want the longest runtime from what's already in the generator's tank.

Many owners use both: propane for storage and starting, gasoline when they need full output or when propane runs short. Running the generator on gasoline for a few minutes every month or two, with stabilized fuel, keeps the carburetor wet and ready.

Safety is the same either way

Fuel choice doesn't change the biggest danger. Both fuels produce carbon monoxide. The U.S. Consumer Product Safety Commission says to operate portable generators outside only, far away from the home, and never inside a home, garage, basement, crawlspace or shed, even with doors and windows open. Point the exhaust away from windows and doors and install battery or battery-backup CO alarms inside. Propane exhaust is not "clean" enough to run indoors.

Read our guide to generator carbon monoxide safety before the next outage, and never refuel or swap a tank while the engine is hot or running.

Quick reference

• Gasoline: about 120,214 Btu per gallon. Propane: about 91,452 Btu per gallon (EIA).
• Expect roughly 9 to 11 percent fewer running and starting watts on propane.
• A 20-pound tank holds about 4.7 gallons and runs a 7,500 to 9,500-watt generator about 5 to 7 hours at half load.
• Propane stores indefinitely in a sound cylinder. Gasoline needs stabilizer and rotation.
• Size your loads against the propane rating if propane is your backup plan.