Typical Fuel Consumption by Generator Size
| Generator Size | Gasoline (50% load) | Propane (50% load) | Natural Gas (50% load) |
|---|---|---|---|
| 5,000 W portable | ~0.75 gal/hr | ~0.9 gal/hr | ~50 cu ft/hr |
| 10,000 W portable | ~1.5 gal/hr | ~1.8 gal/hr | ~100 cu ft/hr |
| 14 kW standby | — | ~2.1 gal/hr | ~140 cu ft/hr |
| 20–22 kW standby | — | ~3.0 gal/hr | ~200 cu ft/hr |
Natural gas standby units connected to a piped supply have no practical runtime limit — the main constraint is generator maintenance intervals, not fuel. Propane standby units are limited by tank size: a 500-gallon tank typically lasts 5–7 days at 50% load on a 20–22 kW unit.
How Fuel Consumption Is Calculated
Fuel consumption scales with generator size (larger engines burn more fuel to produce more power) and with load level (a generator working near its rated capacity burns proportionally more fuel per hour than one running at a light load). This calculator applies a load multiplier to a base per-kW consumption rate for each fuel type, then divides your tank size by the resulting gallons-per-hour rate to estimate runtime. For natural gas, which draws from an effectively unlimited piped supply rather than a tank, the calculator reports consumption in cubic feet per hour instead of a runtime limit.
Two things this calculator can't see: your specific generator model's actual fuel curve (which varies by engine efficiency and can differ meaningfully between brands and even between models of the same size), and real-time local fuel pricing. Check your generator's spec sheet for its exact consumption rate at 50% and 100% load if you have it — it will be more accurate than this general estimate.
Choosing a Fuel Type: Gasoline vs. Propane vs. Natural Gas vs. Diesel
Gasoline is the most widely available fuel and the default for most portable generators, but it has a shelf life of roughly 3–6 months without a stabilizer, degrades faster in heat, and isn't practical for extended multi-day outages without a large stored supply or repeated trips to a gas station that may itself be without power. Propane stores essentially indefinitely, burns cleaner than gasoline, and can be sized with a large dedicated tank for genuine outage independence — the tradeoff is slightly lower energy density per gallon and the upfront cost of a tank if you don't already have one. Natural gas is the most convenient option for a standby system already tied to your home's existing gas line, since there's no tank to monitor or refill, but it depends entirely on utility gas service staying pressurized and isn't an option for portable use. Diesel generators are typically the most fuel-efficient per kWh produced and diesel fuel stores longer than gasoline, but diesel generators cost more upfront and are less common in residential-scale portable and standby units.
A Worked Example
A 14 kW propane standby generator running at 50% load during a 4-day outage, with propane at $3.00/gallon:
Consumption rate: roughly 2.5 gal/hr at 50% load
Cost per hour: 2.5 gal × $3.00 = $7.50/hr
Cost per day (24 hrs): $7.50 × 24 = $180/day
Cost over the 4-day outage: $180 × 4 ≈ $720 in propane, plus roughly 240 gallons consumed — worth checking against your tank size before the outage starts.
Frequently Asked Questions
A generator engine burns fuel roughly in proportion to how hard it's working, but not in a straight line — running at 25% load burns noticeably less than a quarter of what it burns at full load, while running near 100% load burns proportionally more per kWh delivered. That's why this calculator uses a separate multiplier for each load level rather than a flat rate.
It depends on your local propane and natural gas prices, which vary significantly by region and by season. In general, piped natural gas tends to have a lower per-unit fuel cost than delivered propane, but propane systems don't depend on utility gas-line pressure and can be sized with a larger dedicated tank for extended-outage independence.
That depends on your generator's size, your expected load level, and how many days of runtime you want. Enter your generator's kW rating, propane as the fuel type, and your expected outage length in this calculator to see an estimated total gallons consumed, which you can compare against common residential propane tank sizes (120, 250, 500, and 1,000 gallons).
For practical purposes, yes, as long as your utility's gas service and pressure hold up and your home's gas line is sized correctly for the generator's demand — which a qualified installer needs to verify, since an undersized line can starve the generator of fuel under load. The real constraint on a natural gas standby unit's runtime is scheduled maintenance, not fuel supply.
No — actual fuel consumption varies by engine design, generator age, ambient temperature, and altitude. These are general estimates based on typical output-to-fuel-consumption relationships. Your specific generator's owner's manual or spec sheet will list its actual fuel consumption curve, which is the most accurate source if you have it.