Generator Run Time Calculator
Diesel, Gasoline & Propane
Estimate how long your generator will run on gasoline, propane, or diesel based on fuel capacity and hourly consumption rate, with outage-coverage thresholds.
This tool estimates runtime from fuel amount and burn rate. To size your generator for a load, start with the Generator Size Calculator.
Calculate generator runtime
Enter the usable fuel amount and the generator fuel burn rate. This calculator estimates total runtime in hours and days.
Based on 20 gallons of fuel and a burn rate of 0.5 gallons per hour.
Tips for success
- Load matters. Generator fuel consumption changes with electrical load; most manufacturer numbers assume 50% load.
- Add a safety margin. Real runtime is often shorter than ideal estimates. Add a 15–25% safety margin.
- Never run dry. Leaving a reserve margin is wise; never run a tank dry as air can enter the fuel system.
- Rotate your fuel. Fuel age, temperature, and storage conditions matter. Old gasoline can cut runtime and foul carburetors.
- Test before you need it. Test your real-world runtime with a known load before relying on the number in an emergency.
Manufacturer runtime specs are typically stated at 25% or 50% load under lab conditions. At 100% load, expect 30–50% less runtime than the rated number. Gasoline also degrades quickly; stored fuel more than 6 months old without stabilizer can burn 10–20% less efficiently and damage the carburetor. Rotate fuel regularly and treat stored gasoline with a stabilizer.
How long generators actually run (real-world planning)
Generator runtime depends heavily on electrical load. Most generators use significantly more fuel when running closer to their maximum capacity.
- Light load (25%): longest runtime, lowest fuel consumption
- Medium load (50%): realistic planning scenario
- Heavy load (75–100%): much shorter runtime
For accurate planning, always estimate runtime based on how much of your generator capacity you will actually use.
Plan your full system
Gasoline vs propane vs diesel: which fuel runs longer?
Each fuel trades off energy density, storage life, and cold-weather behavior. The figures below use US Energy Information Administration per-gallon energy content and typical burn rates for a 5,000-watt generator at about 50% load.
| Fuel | Energy per gallon | Typical burn (5 kW, 50% load) | Practical shelf life | Cold weather | Best for |
|---|---|---|---|---|---|
| Gasoline | ~120,000 BTU | ~0.5 gal/hr | 3 to 6 months (12+ with stabilizer) | Starts easily | Short outages, portable units, easy availability |
| Propane | ~91,500 BTU | ~1.5 to 2 lb/hr (a 20-lb tank lasts 10 to 12 hr) | Indefinite in sealed tanks | Loses pressure below about 20°F | Clean long-term storage, dual-fuel units, infrequent use |
| Diesel | ~137,000 BTU | ~0.4 gal/hr | 6 to 12 months (longer with biocide) | Harder cold starts | Long outages, standby and commercial units, best runtime per gallon |
Diesel delivers the most runtime per gallon and the best bulk-storage economics, propane stores the longest with the cleanest handling, and gasoline is the most available and easiest to start in the cold. Dual-fuel generators let you keep propane on hand for long-term reserves and fall back to gasoline when you need it.
Diesel generator runtime: how long does a tank actually last?
Diesel runs longer per gallon than gasoline or propane, which is why long-outage and commercial-class generators almost always run on diesel. The trade-off is higher upfront cost and harder cold-weather starting.
Why diesel runs longer per gallon
Diesel stores about 137,000 BTU per gallon, compared with roughly 120,000 for gasoline and 91,000 for propane (US Energy Information Administration standard values). That is only about 14 percent more energy per gallon than gasoline, but diesel engines also burn that fuel more efficiently (typical thermal efficiency of 35 to 45 percent versus 25 to 30 percent for gas), so more of the energy in the tank does useful work. The combined effect is roughly 30 to 40 percent more runtime per gallon than gasoline at the same load.
Typical diesel burn rates
- 5–7 kW class (residential portable): 0.3 to 0.5 gal/hr at 50% load.
- 10–15 kW class (whole-home or large jobsite): 0.7 to 1.1 gal/hr at 50% load.
- 20–30 kW class (commercial standby): 1.4 to 2.0 gal/hr at 50% load.
- 50 kW+ class (light commercial standby): 3.0+ gal/hr at 50% load.
Heavier loads push burn rate up roughly linearly. A 7 kW unit at 100% load typically burns 0.6 to 0.8 gal/hr.
Diesel vs gasoline runtime: a worked example
Take an 8-gallon tank running at 50% load on a 6,500-watt generator. A gasoline unit typically burns 0.55 gal/hr in this range, giving roughly 14.5 hours of runtime. A diesel unit at the same output burns about 0.4 gal/hr, giving about 20 hours of runtime, a 38% gain. Over a 72-hour outage that gap means three or four fewer refueling stops. Diesel also stores longer in sealed containers (up to 1 year vs 6 months for untreated gasoline), so the same tank holds more usable fuel six months into hurricane season.
FAQ
How accurate is this runtime estimate?
The estimate assumes constant fuel consumption at your entered rate. Real runtime is typically 10 to 25 percent shorter because consumption rises with load, fuel tanks drain slower near the bottom, and real conditions (cold weather, altitude, fuel age) all shave time. Use the result as an upper bound and plan to refuel before the tank is empty.
What load should I assume when planning runtime?
Manufacturer runtime specs are usually stated at 25 or 50 percent load in lab conditions. For realistic planning, use the consumption rate at your expected load, or assume 50 percent if you don't know. At 100 percent load, expect 30 to 50 percent less runtime than the rated number.
How much fuel should I store for a long outage?
A reasonable target is enough to run your planned load for 72 hours, since most outages are resolved within that window. For a 5,000-watt generator running 8 to 12 hours per day, that's roughly 15 to 25 gallons of gasoline, 12 to 18 gallons of propane (or one to two 20-lb tanks), or 8 to 15 gallons of diesel. Stagger usage and rotation to stretch reserves.
Does fuel type change how long my generator runs?
Yes. For a given gallon, diesel stores the most energy (about 137,000 BTU/gal), gasoline is next (about 120,000 BTU/gal), and propane is third (about 91,500 BTU/gal), using US Energy Information Administration values. Propane also loses efficiency in cold weather below 20°F. Dual-fuel generators let you switch between gasoline and propane depending on what's available and how long you need to run.
Why is my real-world runtime always shorter than the spec?
Three reasons: manufacturer ratings use fresh fuel at partial load in controlled temperatures, real outages run generators at higher load under variable conditions, and stored fuel loses potency over time. Gasoline stored over 6 months without stabilizer can burn 10 to 20 percent less efficiently. Rotate your fuel and treat it with a stabilizer if you're storing it.
How long will a generator run on 5 gallons?
It depends on the burn rate, which varies by generator size and load. A 2,000-watt inverter at 50% load uses about 0.15 gallons per hour, so 5 gallons lasts roughly 33 hours. A typical 5,000-watt portable burns 0.5 gallons per hour at 50% load, giving 10 hours on 5 gallons. A 7,500-watt unit at full load can burn 1 gallon per hour, dropping that to 5 hours. For a fuller breakdown by tank size, see our guide on how long a generator runs on X gallons.
How much fuel does a generator use per hour?
A reasonable rule of thumb is 0.1 to 0.2 gallons per hour per 1,000 watts of running load on a gasoline generator. A 5,000-watt generator at half load typically burns around 0.5 gallons per hour. Diesel units at the same load burn roughly 0.3 to 0.4 gallons per hour. Propane runs about 1.5 to 2 pounds per hour for that same 5,000-watt-at-half-load case (a 20-pound tank lasts about 10 to 12 hours).
How long will a diesel generator run?
Diesel generators usually run longer per gallon than gasoline because diesel stores about 14% more energy per gallon and diesel engines convert more of that energy to electrical output. A typical 6,500-watt diesel unit with an 8-gallon tank runs about 20 hours at 50% load, versus roughly 14.5 hours for a gasoline equivalent. Larger commercial diesel standby units with 50–100 gallon belly tanks can run continuously for 50 to 100+ hours at moderate load, which is why hospitals and data centers rely on them for extended outages.
Sources & references
Fuel energy-content figures use US Energy Information Administration standard values. Outage-coverage targets follow FEMA and Ready.gov preparedness guidance.
- US Energy Information Administration, Energy units and calculators explained (per-gallon energy content of gasoline, diesel, and propane).
- US Department of Energy, Alternative Fuels Data Center, Fuel Properties Comparison.
- FEMA and Ready.gov, Build A Kit (72-hour, three-day preparedness standard).