A generator runtime calculator estimates how long your generator will run before running out of fuel, based on tank size, fuel type, rated wattage, and the percentage of load you're running. Whether you're preparing for a hurricane, power outage, or camping trip, knowing your generator's fuel autonomy helps you plan refueling schedules and stock the right amount of fuel.
Generator Specs
Rated (continuous) output — check your generator's nameplate
Gasoline is most common for portable generators
Typical portable generators: 1–8 gallons. Standby generators: 100–500 gallons.
Appliance Load Builder
Optional — adds to load aboveAdd appliances to automatically calculate your total load. Checking an appliance updates the load slider above.
How to Use the Generator Runtime Calculator
When a hurricane, winter storm, or grid failure hits, knowing exactly how long your generator will run — and at what cost — is essential for planning. This free generator runtime calculator takes the guesswork out of fuel planning by estimating runtime, fuel consumption rate, and operating costs based on your generator's real specifications.
Step 1: Choose Your Unit System
Toggle between Imperial (gallons, watts) and Metric (liters, kilowatts) at the top. All inputs and results update instantly. This makes the tool useful for generators worldwide, whether you're working in gallons per hour or liters per hour for fuel consumption.
Step 2: Enter Generator Specs
Select your generator's rated wattage from the preset list — common portable generators range from 1,000W to 10,000W, while standby whole-home generators typically run from 10,000W to 20,000W or more. You can also type a custom wattage directly. This is the continuous rated output from your generator's nameplate — not the peak or surge rating. Then enter your fuel tank size in gallons or liters.
Step 3: Select Fuel Type
Gasoline is most common in portable generators. Diesel generators are more fuel-efficient — they produce more power per gallon but are less common for portable units. Propane (liquid propane / LP) burns cleanly and stores indefinitely, but you need more volume for the same energy output. The calculator adjusts fuel consumption rates automatically for each fuel type based on standard energy density values.
Step 4: Set Your Load
Use the load slider or percentage buttons (25%, 50%, 75%, 100%) to set what fraction of the generator's rated output you're actually using. Running at 50% load is optimal — the generator uses noticeably less fuel than at 100%, while producing adequate power. At 100% load, fuel consumption is highest and the generator runs hotter and louder.
Step 5: Use the Appliance Load Builder
Not sure what your load percentage is? Use the appliance load builder to check off the devices you plan to run. Each appliance shows its typical running wattage. The calculator adds them up, compares to your generator's rating, and can automatically set the load slider. Common emergency appliances include refrigerators (150W), window AC units (1,200W), sump pumps (750W), lights (50–200W), and phone chargers (20W). Hit "Apply Appliance Load" to sync with the main calculator.
Step 6: Review Results
The results show estimated runtime in hours, fuel consumption rate in gallons (or liters) per hour, and operating cost per hour based on current fuel prices. The runtime comparison table shows how your runtime changes at 25%, 50%, 75%, and 100% load — useful for deciding which appliances to power down to extend your fuel supply through a multi-day outage. Always keep at least 10% fuel in reserve to avoid damaging the engine from running dry.
Understanding Generator Fuel Consumption
Generator fuel consumption scales with load. The standard industry formula for gasoline generators is approximately: fuel_rate (gal/hr) = (load_watts × heat_rate) / fuel_energy_density. At 50% load, a 5,500W gasoline generator consumes roughly 0.4–0.6 gallons per hour, giving 10–15 hours from a 6-gallon tank. Diesel and propane generators use the same formula with different energy density constants, explaining why diesel typically offers longer runtime per gallon.
FAQ
Is this generator runtime calculator free?
Yes, completely free with no signup required. Calculate runtime for any generator size, fuel type, and load combination. All calculations run locally in your browser — no data is ever sent to a server.
Is my data private when I use this tool?
Absolutely. All calculations run entirely in your browser using JavaScript. No generator specs, appliance lists, or personal information is transmitted anywhere. You can use it offline once the page loads.
How accurate are generator runtime estimates?
These estimates are based on manufacturer-published fuel consumption rates at different load percentages. Real-world runtime varies based on altitude, temperature, generator age, fuel quality, and how well the load matches the rated wattage. Use these figures as planning estimates, then verify with your specific generator's manual.
Why does load percentage affect runtime so much?
Generators are most fuel-efficient at 50-75% load. At 25% load, the engine runs inefficiently and consumes nearly as much fuel as at 50% load while producing less power. At 100% load, fuel consumption spikes. Running your generator at 50-75% is the sweet spot for balancing runtime, efficiency, and engine longevity.
What is the difference between rated watts and surge watts?
Rated (running) watts is the continuous power a generator can supply. Surge (starting) watts is the brief peak needed to start motor-driven appliances like refrigerators, air conditioners, and sump pumps — usually 2-3 times the running wattage. This calculator uses running watts. Make sure your total running load stays below the rated watts, with headroom for surge loads.
How long does a 5-gallon generator tank last?
It depends entirely on the generator size and load. A 2,000W generator at 50% load (1,000W) might use 0.5 gallons per hour, giving you 10 hours from 5 gallons. A 10,000W generator at 75% load uses roughly 1.5 gallons per hour, giving only 3.3 hours. Use this calculator with your specific generator wattage and load to get an accurate estimate.
Can I run my air conditioner on a generator?
Yes, but air conditioners have high starting surge and running wattage. A central 3-ton AC draws 3,500W running and may surge to 9,000W at startup. A portable 10,000 BTU window unit draws about 1,200W. Always account for both surge and running watts when sizing your generator, and avoid running AC at 100% load alongside other heavy appliances.
What fuel type lasts longest in a generator?
Diesel generators are generally most fuel-efficient, giving more kWh per gallon than gasoline. Propane burns cleaner and stores indefinitely, but produces fewer BTUs per unit volume, so you need more of it. Gasoline is most common for portable generators. For whole-home standby generators during extended outages, propane or natural gas offer the most convenient long-term operation.