Whole House Generator GuideNatural gas and propane are the two common fuels for a permanently installed standby generator, and the choice affects power output, runtime, storage, and cost. The cited sources here separate BTU energy density, the natural-gas derate, the pipeline versus tank supply model, storage and shelf life, and operating cost so the trade-off is visible rather than assumed.
Energy density: BTU per cubic foot
The physical difference between the two fuels is energy per unit of volume. Briggs & Stratton states that one cubic foot of propane yields 2,516 BTUs while one cubic foot of natural gas provides 1,030 BTUs. Baker Brothers gives the same relationship in rounded terms, listing propane at roughly 2,500 BTU per cubic foot and natural gas at roughly 1,000 BTU per cubic foot, and iblighting cites about 2,500 BTU per cubic foot for propane versus 1,030 for natural gas.
That gap is the root cause of most of the differences that follow. Because propane packs more than twice the energy into the same volume, an engine drawing natural gas has to move far more fuel to make the same power. The BTU figures are not a marketing preference; they are the starting point for the derate, the consumption rates, and the runtime math covered below.
The natural-gas power derate
Standby generators usually carry two output ratings because the same engine makes less power on natural gas. Baker Brothers notes most standby units carry two output ratings, one for propane and a lower one for natural gas, with propane producing slightly more power due to higher energy density. iblighting quantifies the gap, stating the derate typically falls between 5 and 10 percent across residential air-cooled product lines, and gives an example where a generator producing between 7 kW and 20 kW on propane might yield up to 18 kW on natural gas. Briggs & Stratton shows the same example figures, with a propane range of 7 kW to 20 kW versus up to 18 kW on natural gas.
Across fuels the ranking is consistent. Champion Power Equipment states that propane generally produces slightly less power than gasoline, often around 5 to 10 percent lower, and that natural gas typically results in an even larger reduction in wattage. Grenergy frames the propane side as an efficiency figure, describing propane as 80 percent efficient compared to gasoline, meaning it uses 20 percent more fuel by volume to generate the same horsepower, and attributes the horsepower reduction to mechanical limitations in the fuel system rather than the fuel alone.
Supply and runtime: pipeline versus tank
The supply model is where natural gas has its clearest advantage. Baker Brothers states a natural gas unit runs as long as the utility line supplies gas, with no tank to empty, while a propane unit runs until the tank runs low, with tank size setting the limit and homes often using 250 to 1,000 gallons. Briggs & Stratton notes natural gas pipelines are accessible to about 70 percent of US homes, while propane may be the preferred choice in remote areas due to its portability. Champion Power Equipment adds that a natural gas generator connected to a residential gas line can run without requiring fuel refills.
Consumption rates show how much fuel each side moves. Briggs & Stratton states a 10-kilowatt generator typically consumes about 2.2 gallons of propane per hour, while an average 10 kW generator on natural gas burns roughly the equivalent it lists as 1.5 gallons per hour. iblighting gives larger-unit figures, with a 22 kW Generac consuming 2.1 gallons per hour of propane at half load and 3.6 gallons per hour at full load, and natural gas consumption of 203 cubic feet per hour at half load and 306 cubic feet per hour at full load.
Storage and shelf life
Propane and natural gas differ sharply in how they are stored. Champion Power Equipment states that propane does not degrade over time and that propane tanks and larger storage tanks can be stored for extended periods, while natural gas requires access to a home or commercial gas supply line with no on-site storage. iblighting describes propane as having an indefinite shelf life when stored properly, and describes natural gas as flowing continuously from the utility with no tank to store fuel on site.
That storage difference drives a control trade-off during emergencies. Baker Brothers notes propane can be stored for years, and that a homeowner can fill the tank before a storm to control the supply, while natural gas can be cut off if a gas line is damaged in a major disaster. So propane trades the convenience of a pipeline for on-site fuel the owner physically controls, and natural gas trades stored reserve for a hands-off supply that depends on the utility staying intact.
Operating cost and availability
On a per-unit basis natural gas is generally the cheaper fuel. Baker Brothers states natural gas is often cheaper per unit of fuel than propane. iblighting puts numbers to it, citing residential propane averaging 2.43 dollars per gallon nationally and residential natural gas typically running 0.90 to 1.30 dollars per therm, or roughly 9 to 13 dollars per thousand cubic feet. For a 22 kW generator at full load, iblighting estimates a propane operating cost of roughly 325 to 440 dollars per year against a natural gas cost of roughly 125 to 165 dollars per year.
Availability cuts the other way for homes without a gas line. Briggs & Stratton notes natural gas pipelines reach about 70 percent of US homes, leaving propane as the option for the rest, and describes propane as the preferred choice in remote areas due to its portability. So the cost advantage of natural gas only applies where the pipeline already exists, and propane remains the practical fuel where it does not. Briggs & Stratton also notes its NGMax technology as an approach intended to let natural gas units match propane power output, addressing the traditional energy density gap.
Frequently asked questions
Why does a generator make less power on natural gas?
Natural gas holds far less energy per cubic foot than propane. Briggs & Stratton lists 1,030 BTU per cubic foot for natural gas versus 2,516 for propane, and iblighting says the resulting derate typically runs 5 to 10 percent on residential air-cooled units.
How much less energy does natural gas hold than propane?
Briggs & Stratton states one cubic foot of propane yields 2,516 BTUs while one cubic foot of natural gas provides 1,030 BTUs, so propane holds more than twice the energy per unit of volume.
Which fuel runs longer during an outage?
Baker Brothers says a natural gas unit runs as long as the utility line supplies gas with no tank to empty, while a propane unit runs until its tank, often 250 to 1,000 gallons, runs low.
Is natural gas or propane cheaper to run?
iblighting estimates a 22 kW generator at full load costs roughly 325 to 440 dollars per year on propane versus 125 to 165 dollars per year on natural gas, and Baker Brothers notes natural gas is often cheaper per unit.
Which fuel stores better?
Champion Power Equipment and iblighting note propane does not degrade and has an indefinite shelf life when stored properly, while natural gas has no on-site storage and flows continuously from the utility line.