How Much Gas Does a Generator Use? Consumption by Size and Load

Fuel consumption is driven by how much power you're drawing, not by the generator's rating. Understanding that changes how much fuel you need to store.

How much gas does a generator use per hour?

Gasoline, at approximately 50% load:

At full load, expect meaningfully higher — often approaching double the half-load figure.

Manufacturers publish consumption at specified load points, usually 25%, 50%, and 100%. Those figures are far more reliable than any general table, including this one. Check your manual.

Estimate for your own scenario with the generator runtime calculator.

Why does an underloaded generator waste fuel?

Generators aren't equally efficient across their range. Most are best somewhere around 50–75% of rated load.

Very light load wastes fuel per unit of output. A conventional generator runs at fixed speed regardless — 3,600 rpm whether you're drawing 500 W or 5,000 W — so idling a large generator to power a lamp is genuinely wasteful.

Full load raises consumption disproportionately and stresses the engine.

This is where inverter generators have a real advantage: they throttle the engine to match demand, so light loads genuinely burn less fuel. It's also why they're quieter at low load. See what is an inverter generator.

Practical implication: don't buy far more generator than you need if fuel efficiency matters. A 10,000 W unit powering 1,500 W of essentials is running well below its efficient range.

Fuel by type

Gasoline — highest energy density, most output, shortest storage life. See gasoline storage and stabilizer.

Propane — roughly 10% less output, and consumption is usually quoted in pounds or gallons of propane per hour. Stores indefinitely, which is the real argument. See dual fuel and tri-fuel generators.

Natural gas — lower output again, but the supply is continuous and typically keeps flowing during electrical outages. Consumption is metered rather than stored, so "how much fuel do I need" stops being a question. See natural gas vs propane generator.

How do you convert propane and natural gas figures?

Gasoline is simple because the spec sheet and the pump use the same unit. The other two need a conversion, and it's where most people give up.

Propane gets quoted in pounds or gallons per hour depending on the manufacturer:

• Propane weighs about 4.24 lb per gallon
• It carries about 91,500 BTU per gallon
• So a "20 lb" barbecue cylinder holds roughly 4.7 gallons — around 430,000 BTU

That last figure is the useful one. A generator burning 1 gal/hour of propane empties a 20 lb cylinder in under five hours. This is why barbecue cylinders are a stopgap and 100 lb cylinders (about 23.6 gallons) or a fixed tank are the real answer. Remember tanks fill to about 80%, so a 500-gallon tank holds roughly 400 usable gallons.

Natural gas is specified in cubic feet per hour while your utility bills in therms or CCF:

• 1 therm = 100,000 BTU
• Natural gas runs about 1,000–1,050 BTU per cubic foot
• So 1 therm ≈ 100 cubic feet ≈ 1 CCF

A standby unit consuming 200 ft³/hour is burning about 2 therms/hour. Multiply by your all-in per-therm rate for the hourly cost.

Does altitude change fuel consumption?

Yes, and it's the factor almost nobody accounts for.

Naturally aspirated engines lose power as air thins with elevation — commonly cited at roughly 3–3.5% per 1,000 feet above sea level. A 5,000 W generator at 5,000 feet may only deliver around 4,200 W.

The consumption consequence follows: to produce the same output, the engine has to work at a higher fraction of its (now reduced) capacity, so you burn more fuel per kilowatt-hour than the sea-level table suggests.

Two practical points:

• Size up if you live at elevation. A generator that just covers your load list at sea level may not cover it at 6,000 feet.
• Carbureted engines may need a high-altitude jet kit. Running the sea-level jetting at elevation gives a rich mixture — wasted fuel, sooty plugs and rough running. Check whether the manufacturer offers one for your model, and note that these kits usually shouldn't stay fitted if you bring the unit back down.

Manufacturers publish their own derate figures. Use those over the rule of thumb where you can find them.

How do you measure your own consumption?

Any table, including the one above, is a starting point. Your actual figure depends on your load list, your unit and your elevation — and measuring it takes one afternoon:

1. Fill the tank completely and note the time.
2. Run your real load list — the fridge, the furnace, the router, whatever your plan is — for a measured period, ideally two hours or more.
3. Refill from a measured container and record how much it took.
4. Divide gallons by hours.

Do this once, write the number on a label stuck to the generator, and every future outage plan becomes arithmetic instead of guesswork. It's also the honest test of whether your generator starts and carries your loads — which is worth finding out on a calm Saturday.

One caution on published runtime figures: manufacturers usually quote them at a light load, commonly 25%. A "12 hour runtime" on the box is not 12 hours of running your house.

How much fuel should you store for an outage?

The key insight: most households don't need to run continuously.

A refrigerator holds temperature for hours when closed. A freezer holds considerably longer. Devices charge in an hour. Heat can be intermittent in a well-insulated house.

So a realistic pattern is cycling: run a few hours in the morning, a few in the evening, off overnight. That can halve fuel consumption compared with 24-hour operation, and it gives you quiet nights.

A worked estimate for a 5,000 W generator at moderate load:

For a three-day outage, that's roughly 36, 18, or 9 gallons — a large difference in how much fuel you need to store, and in whether storage is practical at all.

Note that storing 36 gallons of gasoline at home is a lot, may exceed local fire code limits, and creates its own hazards. This is a strong argument for propane, natural gas, or a battery that the generator recharges.

How do you reduce generator fuel use?

Cycle rather than run continuously. The single biggest lever.

Run at a sensible load — not idling a large generator for a small draw.

Use an inverter generator if light loads are typical.

Pair with a battery. Run the generator a couple of hours to recharge a power station, then run silently on battery. Dramatically less runtime and fuel, plus quiet nights. See battery backup vs generator.

Reduce the load itself — LED lighting, keep the fridge closed, skip the electric kettle.

Storage limits

Before planning around a large fuel reserve:

• Local fire codes limit stored quantities, and limits vary
• Storage conditions matter — approved containers, detached ventilated space, away from ignition sources
• Gasoline degrades, so a large reserve needs rotating

Propane sidesteps all three: it stores indefinitely, and larger tanks are a normal residential fixture. See propane tank sizing for a generator.

Where to go next

• Cost to run a generator
• How long will a generator run?
• Generator fuel types

More in our generator and backup power guides.