How Much Power You Lose Running a Generator on Propane or Natural Gas

If you’ve ever compared a dual-fuel generator’s gasoline wattage rating to its propane or natural gas rating and noticed the numbers don’t match, you’re not misreading the spec sheet — portable generators genuinely produce less power on gaseous fuel. Here’s exactly how much, why it happens, and how it’s different for standby generators.

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The Numbers: How Much Power You Actually Lose

On a portable dual-fuel generator — one primarily designed and rated for gasoline, with propane or natural gas as an add-on option — the derate is real and worth planning around:

• Propane: typically derates a generator’s gasoline rating by roughly 10%.
• Natural gas: typically derates a generator’s gasoline rating by roughly 20–25% — a deeper loss than propane.

A concrete example: a popular 7,500W dual-fuel portable generator publishes both a 7,500W gasoline running rating and a 6,750W propane running rating — almost exactly a 10% derating in practice, matching the rule of thumb above.

Why This Happens

The mechanism comes down to how each fuel enters the cylinder. Gasoline is a liquid that vaporizes into a dense, energy-rich air-fuel charge as it’s drawn into the engine. Propane and natural gas are already gaseous at the point they enter the intake — which means they physically displace some of the air that would otherwise be pulling in with the fuel-air mixture, resulting in a marginally less energy-dense charge in the same cylinder volume. Natural gas derates further than propane because it also has a lower BTU content per unit volume than propane, compounding the displacement effect with a genuinely lower-energy fuel.

This Matters Most When Sizing a Portable Generator

If you plan to run a dual-fuel portable generator on propane or natural gas during an actual outage — rather than gasoline — size your load against the derated figure, not the gasoline number on the box. A generator that comfortably covers your essential circuits at its 7,500W gasoline rating may only deliver 6,750W on propane, which can be the difference between running your fridge, well pump, and a few circuits comfortably versus tripping an overload breaker mid-outage. Do this math before you buy, not after a generator underperforms during a storm.

Standby Generators Work Differently

This derate is specific to portable dual-fuel units built primarily around gasoline. Whole-house standby generators (the Generac Guardian line and similar) are factory-designed and tuned for gaseous fuel from the start — they never run on gasoline at all, so there’s no gasoline baseline to derate against. Between propane and natural gas specifically on a standby unit, the gap is much smaller: liquid propane typically gets the generator’s full rated output, while natural gas commonly runs a modest few-percent derate relative to the propane rating, varying by manufacturer and model — see our Generac 22kW vs 26kW comparison for a real example of how this plays out on a specific model line. Don’t apply the portable-generator 10%/20-25% figures to a standby unit — they’re a different category of derate entirely.

Elevation and Temperature Also Derate Output

Fuel type isn’t the only factor. Most engines lose roughly 3–4% of rated output for every 1,000 feet of elevation above sea level, due to thinner air reducing the oxygen available for combustion. Extreme heat can also modestly reduce output. If you live at meaningful elevation and plan to run on propane or natural gas, both derates stack — worth accounting for both when sizing a generator for your specific situation rather than relying on the sea-level gasoline number on the box.

A concrete example of how these stack: a generator rated 7,500W on gasoline at sea level might realistically deliver something closer to 6,750W on propane (the roughly 10% fuel derate) before elevation is even considered. Add a mountain-town elevation of 5,000 feet (roughly 15–20% additional loss at that height) and the real-world output drops meaningfully further than the number on the box would suggest — which is exactly why sizing conversations with a dealer in higher-elevation regions should always start from your actual expected fuel and elevation, not the gasoline-at-sea-level headline spec.

Practical Buying Advice

When shopping for a dual-fuel or tri-fuel portable generator, always check the manufacturer’s spec sheet for separate running-watt and starting-watt ratings by fuel type — not just the headline gasoline number most retailers feature prominently. See our natural gas vs. propane comparison for the fuller tradeoff beyond just output, and consider a clamp meter if you want to verify your actual circuit draw against the derated rating before an outage hits.

Key Takeaways

• Portable dual-fuel generators lose roughly 10% of their gasoline-rated output on propane, and roughly 20–25% on natural gas.
• This happens because gaseous fuel entering the cylinder displaces intake air compared to vaporized gasoline, and natural gas has a lower energy content than propane, compounding the loss.
• Always size your outage load against the derated wattage figure, not the gasoline number printed largest on the box.
• Standby generators (Generac Guardian and similar) are factory-tuned for gaseous fuel and don’t carry this same derate — the propane-to-natural-gas gap on standby units is much smaller.
• Elevation (roughly 3–4% per 1,000 ft) and extreme heat also reduce output and stack with the fuel-type derate.

Frequently Asked Questions

Why don’t manufacturers just rate portable generators for their propane output instead of gasoline?
Most buyers shop by the highest number on the box, and gasoline produces the highest output on a dual-fuel unit, so it’s typically the headline spec. The derated propane and natural gas figures are usually in the fine print or spec sheet rather than the box art.

Is the derate the same across all portable generator brands?
The roughly 10%/20-25% figures are a reasonable rule of thumb, but exact derating varies by engine design and carburetor tuning. Always check the specific model’s published running-watt ratings by fuel type rather than assuming the rule of thumb applies exactly.

Does a tri-fuel generator lose power on all three fuels equally?
No — gasoline is the baseline (0% derate, since it’s what the engine is primarily tuned for), propane loses roughly 10%, and natural gas loses roughly 20–25%. Check your specific model’s spec sheet for exact numbers on all three fuels before relying on it for a specific load.