How Long Will a Generator Run? Runtime & Fuel Use

How long a home generator runs comes down to three things: how much fuel it can draw on, how hard you're loading it, and what type of fuel it burns. There's no single number — the same generator can run for hours or for days depending on those factors.

Here's the short version. A mid-size gasoline portable (5,000–7,000W) typically runs about 8–10 hours on a 5–8 gallon tank at a moderate (roughly half) load. Lighten the load and it runs longer; push it toward full output and it runs shorter, because fuel consumption rises with load — a generator at half load burns far less per hour than the same unit flat out. Propane and diesel standby units running off large tanks can last days, and a natural-gas standby tied to your utility line runs essentially indefinitely. Below is how to estimate your own runtime and what it costs to keep the lights on.

What determines generator runtime

Three variables set how long any generator runs:

1. Fuel supply. A portable carries its fuel in an onboard tank, typically 3–9 gallons. A standby unit draws from a large external source — a 250-gallon propane tank, a diesel day-tank, or an unlimited natural-gas line.
2. Load. The total wattage you're powering. The more watts you pull, the more fuel the engine burns per hour. This is the single biggest lever you control.
3. Fuel type. Gasoline, propane, natural gas, and diesel differ in energy content, efficiency, and how they're stored — which changes both runtime and how long the fuel itself lasts in storage.

Get a handle on these three and you can estimate runtime for almost any setup.

Runtime and fuel use by load

Fuel consumption isn't flat — it climbs as you add load. The table below shows rough planning figures for a typical mid-size portable (about 5–7 kW) with a representative 6-gallon tank. Treat these as illustrative estimates; your unit's spec sheet has the real numbers for its specific engine.

The pattern is the clear takeaway: running at half load instead of full load can nearly double your runtime on the same tank. During an outage, powering only what you need — the refrigerator, a few lights, a furnace blower — stretches your fuel much further than running every circuit at once.

Runtime by fuel type

Different fuels change the runtime math, the storage story, and how you refuel.

Gasoline portables

The most common backup setup. A 5,000–7,000W gasoline unit generally runs 8–10 hours on a 5–8 gallon tank at a moderate load. Gasoline is easy to buy, but it's the worst fuel to store: it degrades in a few months (less with stabilizer), so rotate it. During a long outage you'll likely refuel — always shut the unit down and let it cool first, because adding gas to a hot engine is a fire hazard.

Propane (LP) standby and portables

Propane stores indefinitely without degrading, which makes it popular for standby units. A standby generator on a 250-gallon tank (about 200 usable gallons) can run for days — light loads stretch it dramatically further than heavy ones. Propane burns slightly less efficiently than gasoline per gallon, but the long shelf life and clean burn are why many whole-home systems use it. Small portables can also run on 20-lb (5-gallon) BBQ-style cylinders for shorter stretches.

Natural gas standby

A natural-gas standby connected to your home's utility line runs essentially indefinitely — there's no tank to empty, because the fuel supply is continuous. That's the headline advantage: in a prolonged outage you don't refuel at all. The practical limits become maintenance (oil changes, service intervals) rather than fuel. The catch is that natural gas depends on the utility pipeline staying pressurized, which it usually does even during electrical outages.

Diesel

Diesel engines are fuel-efficient and built to run long hours, which is why they dominate larger standby and commercial units. Diesel also stores well — longer than gasoline, especially treated and kept dry — making it a solid choice for units that sit idle between outages. Expect strong runtime per gallon under load, with the tradeoff being a higher-cost engine and cold-weather gelling to watch in very low temperatures.

Estimating fuel cost

Once you know fuel use per hour, cost is simple arithmetic:

Cost per hour = fuel use (gal/hr) × fuel price per gallon. Cost per day = cost per hour × hours run.

For example, a portable burning 0.6 gal/hr at a moderate load uses about 14–15 gallons over a 24-hour day. Multiply by whatever you pay per gallon to get the daily cost — fuel prices vary widely by region and over time, so plug in your local price rather than relying on a fixed figure. Propane and natural gas are priced differently (per gallon and per therm/CCF respectively), but the same approach applies: consumption rate × unit price.

To size the generator itself before you worry about runtime, see how to size a home generator and run the numbers with the generator sizing calculator.

Safety: the non-negotiables

Generator runtime planning is pointless if the setup isn't safe. Two rules matter most:

• Never run a generator indoors, in a garage, in a basement, or near open windows or vents. The exhaust contains carbon monoxide — colorless, odorless, and deadly. Run it outdoors only, well clear of the house, with the exhaust pointed away from doors and windows. A working CO alarm inside is essential.
• Never refuel a running or hot generator. Shut it off and let it cool before adding fuel. Spilled gasoline on a hot engine or exhaust can ignite.

Also keep the unit dry and on a stable surface, don't overload it past its rated wattage, and follow the manufacturer's maintenance schedule — oil changes especially, since those intervals often become the real limit on how long a standby unit can run continuously.

How to extend your runtime

A few habits make any tank last longer:

• Shed load. Power only essential circuits. Half load instead of full can nearly double runtime.
• Cycle big loads. Run the well pump or space heater in bursts rather than continuously.
• Right-size the generator. An oversized unit lightly loaded is inefficient; a properly sized one running at 40–60% is in its efficient sweet spot.
• Keep fuel on hand and rotated. For gasoline, store stabilized fuel and rotate it. For propane or diesel, a full tank before storm season buys you days.

The bottom line

There's no universal runtime number — it depends on fuel supply, load, and fuel type. A mid-size gasoline portable runs roughly 8–10 hours on a 5–8 gallon tank at moderate load; propane and diesel standby units on large tanks run for days; and a natural-gas standby runs indefinitely off the utility line. The biggest lever you control is load: run only what you need and your fuel goes much further. Always confirm against your unit's spec sheet, plug in your own local fuel price for cost, and never compromise on the carbon-monoxide and refueling safety rules.

For more on choosing the right unit, compare portable vs standby generators and the different generator fuel types, or browse all of our home energy guides.