How long will a generator run on 5 gallons of gas?

Runtime on 5 gallons of fuel depends on generator size, how hard you are running it, and how efficient the engine is. This guide breaks down typical runtimes for common generator sizes and load levels so you can plan more realistic fuel needs.

For quick estimates across different fuel amounts and loads, you can also use the runtime and fuel cost calculators on the GeneratorCalc home page.

How long will a generator run on 5 gallons of gas?

A practical overview of what affects runtime on a typical portable generator using roughly 5 gallons of gasoline.

How long a generator can run on 5 gallons of gasoline depends on the generator size, the load you’re putting on it, fuel efficiency, and real-world conditions like temperature and maintenance. In general, most portable gasoline generators will run anywhere from 7 to 14 hours on 5 gallons of fuel, depending on load.

That’s a large range, so it helps to break it down by generator size and load level.

Typical runtime on 5 gallons of gas

The table below shows very rough ranges that many users report for common portable generator sizes. These are not model-specific numbers, but typical real-world experiences.

Generator size	Runtime @ 25% load	Runtime @ 50% load	Runtime @ 75% load
2,000W inverter	12–16 hours	7–10 hours	4–6 hours
3,500W portable	10–13 hours	6–9 hours	4–6 hours
5,000–6,500W portable	9–12 hours	5–8 hours	3–5 hours
8,000W+ generator	6–10 hours	4–6 hours	2–4 hours

Two people can pour in the same 5 gallons of gas and still see very different runtimes simply because their generator size and load level are different.

The biggest factor: load percentage

A generator burns much less fuel at 25% load than it does at 75% or 100%. For example, a mid-sized portable unit might:

Use around 0.35 gallons per hour at a light load
Use around 0.60 gallons per hour at a heavy load
Use around 0.75 gallons per hour when very close to its rated maximum

That kind of difference can easily cut runtime in half. If you want a more precise estimate for your situation, you can enter your tank size and load level into the runtime calculator at the top of this page and get an instant estimate.

A simple runtime formula

If you have a good guess for your generator’s fuel consumption rate, you can estimate runtime with a simple formula:

Runtime (hours) = Fuel amount (gallons) ÷ Fuel consumption rate (gallons per hour)

For example, if your generator burns about 0.5 gallons per hour at the load you’re using, then:

5 gallons ÷ 0.5 = 10 hours of runtime

If you don’t know your fuel rate, the calculator on this page uses typical consumption values for gasoline, diesel, and propane at different load levels as a starting point.

What about propane or diesel?

If you’re using propane or diesel instead of gasoline, the picture changes a bit:

Propane often produces slightly shorter runtimes at higher loads compared to gasoline for the same tank size, but has a long shelf life and stores cleanly.
Diesel generators are often more fuel-efficient and can provide longer runtimes for the same volume of fuel, especially under heavier loads.

The runtime calculator on this page lets you choose gasoline, diesel, or propane to see how the estimate shifts between fuels.

Real-world factors that change runtime

Even with good estimates, your actual runtime can still vary due to:

Temperature: very cold or very hot conditions can change how efficiently the engine runs and how thick the fuel is.
Generator age and maintenance: older engines, dirty air filters, and worn spark plugs can all make a generator burn more fuel than when it was new.
Altitude: at higher elevations, thinner air can reduce engine power, which can affect how the generator behaves under load.
Load spikes: refrigerators, freezers, pumps, and air conditioners can draw extra power when they start.

The advanced options in the runtime calculator are designed to give you a feel for how these kinds of conditions can nudge your estimate up or down.

How to make 5 gallons last longer

You can stretch your runtime by managing your loads carefully:

Turn off large appliances you don’t truly need during an outage.
Use LED lighting instead of older, less efficient bulbs.
Avoid running electric space heaters on a small generator when possible.
Run heavy loads one at a time rather than all at once.
Let refrigerators and freezers cycle, rather than opening them constantly.

When 5 gallons isn’t enough

If you know you’ll need power for an extended period, you may want to consider:

A generator with a larger built-in fuel tank
An extended-run fuel system (where allowed and installed safely)
Dual-fuel units that can run on both gasoline and propane
Using a portable power station for quiet, overnight loads

Bottom line

Most portable generators will run somewhere in the neighborhood of 7 to 14 hours on 5 gallons of gas, depending mainly on generator size and load level. To get a more tailored estimate for your situation, you can use the runtime calculator at the top of this page, plug in your tank size and load, and explore how different conditions might change your results.