Generator Fuel Consumption by Size (Charts)

Fuel consumption is the number that actually determines whether a generator can keep your house running through a multi-day outage — not the kW rating alone. Here’s what standby and portable generators actually burn at different sizes and loads, on both propane and natural gas.

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Why Load Percentage Matters More Than the kW Rating

A generator’s fuel consumption isn’t a fixed number — it scales with how much load you’re actually drawing, and the relationship isn’t perfectly linear. Dropping from full load to roughly half load typically cuts fuel consumption by something closer to 40–45%, not a flat 50% — engines have a baseline fuel draw just to keep running (overcoming internal friction, driving the alternator at idle) that doesn’t scale down proportionally with load. This is part of why sizing a generator closer to your actual essential-circuit load, rather than buying more capacity than you need “to be safe,” pays off in fuel efficiency as well as upfront cost.

Propane Consumption by Generator Size

Approximate propane consumption for air-cooled standby generators, in gallons per hour:

• 10 kW: ~1.0 gal/hr at 50% load, ~1.6 gal/hr at full load
• 14 kW: ~1.4 gal/hr at 50% load, ~2.2 gal/hr at full load
• 18 kW: ~1.6 gal/hr at 50% load, ~2.8 gal/hr at full load
• 22 kW: ~1.7 gal/hr at 50% load, ~3.2 gal/hr at full load
• 26 kW: ~2.1 gal/hr at 50% load, ~3.9 gal/hr at full load
• 30 kW (liquid-cooled): ~2.4 gal/hr at 50% load, ~4.3 gal/hr at full load

As a rule of thumb, expect roughly 0.12–0.15 gallons of propane per rated kW per hour at full load for air-cooled units. See our propane tank sizing guide to translate these hourly rates into how long a given tank size will actually last during an extended outage.

Natural Gas Consumption by Generator Size

Natural gas is measured in cubic feet per hour rather than gallons. Approximate full-load consumption:

• 10 kW: ~120–140 ft³/hr
• 14 kW: ~180–200 ft³/hr
• 18 kW: ~230–250 ft³/hr
• 22 kW: ~280–300 ft³/hr
• 26 kW: ~330–400 ft³/hr
• 30 kW (liquid-cooled): ~380–420 ft³/hr

As a rule of thumb, expect roughly 10–13 cubic feet of natural gas per rated kW per hour at full load. Unlike propane, natural gas draws from your utility line rather than a finite tank, so the practical concern shifts from “how much fuel do I have” to “can my gas meter and line actually deliver this flow rate” — worth confirming with your gas utility or installer before assuming an unlimited supply, especially for larger units.

Which Fuel Actually Costs Less to Run

Cost comparisons shift with local propane and natural gas pricing, which vary significantly by region and season, so there’s no single universal answer — see our natural gas vs. propane comparison for the fuller picture beyond just consumption rate, including reliability during a regional disaster (buried NG lines vs. an on-site propane tank) and installation cost differences.

Portable Dual-Fuel Generators Burn More per kW

These consumption figures are for standby generators, which are factory-tuned for gaseous fuel from the start. Portable dual-fuel generators (rated primarily for gasoline, with a propane or natural gas option added) generally consume more fuel per kW of actual output on gaseous fuel, partly because their rated output itself drops when switched off gasoline — see our generator power derate guide for exactly how much power (and effectively how much fuel efficiency) you lose making that switch on a portable unit.

Estimating Your Own Runtime

To estimate how long your fuel supply will last: identify your typical outage load (not your generator’s max rating — your actual essential circuits), find the closest size/load combination above, and divide your available fuel (propane tank gallons, or confirm unlimited supply for natural gas) by the hourly consumption rate. Building this math into your outage planning, rather than guessing, is the difference between confidently riding out a multi-day outage and running dry on day two.

Liquid-Cooled vs. Air-Cooled Efficiency

Liquid-cooled generators, typically found at 22kW and above, generally deliver somewhat better fuel efficiency per kW of output than air-cooled units of similar size, since a liquid cooling system manages engine temperature more consistently than air cooling alone, allowing for a more stable combustion process. This is one reason larger homes with bigger loads often lean liquid-cooled once they cross the size threshold where both cooling types are available — the fuel savings compound over a multi-day outage, on top of the durability advantages liquid cooling offers for sustained heavy-load operation.

Don’t Forget Tank Refill Lead Time

If you’re running on a propane tank rather than natural gas, factor in how long a refill actually takes to arrange once you’re running low — propane suppliers can be backed up during a widespread regional outage when everyone with a tank is calling at once. Setting up automatic delivery well before hurricane season or winter storm season, rather than waiting until your tank gauge reads low during an active outage, removes one more variable from an already stressful situation.

Key Takeaways

• Fuel consumption scales with load, but not linearly — dropping to half load cuts consumption by roughly 40–45%, not a flat 50%.
• Air-cooled standby generators burn roughly 0.12–0.15 gallons of propane per rated kW per hour at full load, or roughly 10–13 cubic feet of natural gas per rated kW per hour.
• A 22kW standby generator burns approximately 3.2 gal/hr of propane or 280–300 ft³/hr of natural gas at full load; a 10kW unit burns roughly half that.
• Natural gas draws from your utility line rather than a finite tank — confirm your gas meter and line can actually deliver the flow rate for larger generators.
• These figures are for standby generators tuned for gaseous fuel from the factory; portable dual-fuel units consume more per kW of actual output because their rated output itself drops on propane or natural gas.

Frequently Asked Questions

How long will a 100-gallon propane tank last running a 22kW standby generator?
At full load (~3.2 gal/hr), roughly 31 hours. At a more typical 50% load during a partial-house outage (~1.7 gal/hr), roughly 58 hours — just over two days. Real-world runtime will vary with your actual load and outside temperature.

Does cold weather increase fuel consumption?
Yes, moderately — cold starts and cold-engine operation typically burn somewhat more fuel until the engine reaches normal operating temperature, and any electric block heater or battery warmer draws additional fuel or electricity on top of the generator’s own consumption.

Can I run a natural gas generator without worrying about running out of fuel?
Practically yes, since it draws from the utility line rather than an on-site tank, but it’s not risk-free — a natural gas outage (from the same storm that caused your power outage, or damage to the gas line) would still leave you without fuel, which is why some owners choose propane specifically for that independence.