Why Generators Lose 10% on Propane & 20% on Natural Gas
Generators lose approximately 10 percent power on propane and 20 percent on natural gas compared to gasoline. This occurs because gaseous fuels displace oxygen inside the combustion chamber and possess lower volumetric energy density per intake stroke.
01 Volumetric Efficiency and Combustion Chamber Physics
Gasoline is injected or atomized as a liquid mist that occupies virtually zero air volume in the intake manifold, allowing the cylinder to fill with 100 percent air and oxygen.
Natural gas and propane enter the engine as dry gases. The gaseous fuel displaces 10 to 15 percent of incoming combustion air, starving the cylinder of oxygen molecules.
NFPA 54 requires flexible fuel lines to deliver full engine demand with under 0.5 inches Water Column pressure loss. Generators drawing over 85,000 BTU/hr demand minimum 3/4-inch hoses.
- Gaseous fuels displace incoming combustion air in the intake runner.
- Less oxygen per stroke limits maximum cylinder peak combustion pressure.
- Gasoline produces maximum peak mechanical horsepower per engine displacement.
02 Energy Density Comparison (BTU Chemistry)
Gasoline packs 114,000 BTUs per gallon, liquid propane delivers 91,500 BTUs per gallon, and natural gas contains roughly 1,000 BTUs per cubic foot.
Consult our fuel comparison table below to calculate expected running watts across all three fuels on a 10,000-watt generator platform.
| Fuel Type | Energy Density | Power Loss % | 10kW Generator Running Output | Storage Lifespan |
|---|---|---|---|---|
| Gasoline (E10) | 114,000 BTU/gal | 0% (Baseline) | 9,000 Running Watts | 3 to 6 Months |
| Liquid Propane (LPG) | 91,500 BTU/gal | 10% Derate | 8,100 Running Watts | Indefinite (Decades) |
| Natural Gas (Utility) | 1,030 BTU/cu ft | 18% to 22% Derate | 7,200 Running Watts | Continuous Pipe (Infinite) |
NFPA 54 requires flexible fuel lines to deliver full engine demand with under 0.5 inches Water Column pressure loss. Generators drawing over 85,000 BTU/hr demand minimum 3/4-inch hoses.
03 Calculating Your Sizing Safety Buffer
If your home requires 7,500 running watts to operate your central air conditioner and refrigerators, a generator rated at 8,000 watts on gasoline will deliver only 6,500 watts on natural gas.
Always calculate your required appliance wattage and apply the natural gas derating factor using our Derating Waterfall Sizer.
NFPA 54 requires flexible fuel lines to deliver full engine demand with under 0.5 inches Water Column pressure loss. Generators drawing over 85,000 BTU/hr demand minimum 3/4-inch hoses.
- 1 Check Fuel Specification: Look at the generator nameplate for separate Gasoline, LPG, and NG ratings.
- 2 Calculate Load Headroom: Ensure running demand stays under 80% of natural gas capacity.
- 3 Inspect Hose Diameter: Use 3/4-inch natural gas hose to avoid compounding power loss with friction.
- Generators lose 10% power on propane and 20% on natural gas.
- Gaseous fuels displace incoming air, reducing available oxygen for combustion.
- Size generators based on natural gas output when planning whole-home backup.
Frequently Asked Questions
Q: Can I tune my generator carburetor to recover natural gas power?
No. The power loss is fundamentally caused by lower oxygen volume in the cylinder. Advancing ignition timing helps slightly, but the 20% loss remains.
Q: Why buy a tri-fuel generator if it produces less power on natural gas?
Convenience and infinite runtime. Utility natural gas flows continuously through buried pipes, eliminating the need to haul and store volatile gasoline.
Q: Does propane gum up carburetors like gasoline?
No. Propane and natural gas are dry gaseous fuels that never degrade, form varnish, or foul carburetor jets, even when stored for ten years.