How long a generator runs depends on how much fuel you have and how hard it is working. This calculator interpolates the manufacturer’s consumption between known load points and divides your fuel quantity by the resulting hourly rate to estimate total runtime.
How it works
Fuel consumption rises with load, so the tool linearly interpolates between the data-sheet rates at 50 and 100 percent load (or 25 percent when supplied) to find the rate at your operating load, then divides fuel by that rate:
rate(load) = interpolate(consumption table, load %)
Runtime hours = fuel quantity ÷ rate(load)For loads below 50 percent it extrapolates down toward the 25 percent figure where available, otherwise it holds the 50 percent rate as a floor.
Example
A diesel unit burning 0.6 gal/h at 100 percent and 0.4 gal/h at 50 percent, run
at 75 percent, consumes about 0.5 gal/h. A 30-gallon tank then lasts roughly
30 ÷ 0.5 = 60 hours.
Where to find consumption figures
The data sheet that ships with most standby and portable generators lists fuel consumption in gallons per hour at multiple load percentages, typically 25%, 50%, 75%, and 100% of rated output. If only one figure is printed, it is almost always the full-load rate. Manufacturer websites usually publish the full table under “specifications” or “performance data.” For older units without published specs, measuring consumption experimentally over a known run time is the most reliable path.
Fuel type differences
Gasoline generators carry small built-in tanks (typically 2 to 8 gallons on portable units) and are mainly used for short outages. A 5,500-watt portable at 50% load might burn around 0.45 gallons per hour, giving roughly 10 hours from a full tank.
Diesel standby units are more efficient per kilowatt-hour and are usually plumbed to larger storage tanks, making runtime a days-long planning problem rather than an hours problem.
Propane is measured in liquid gallons (or sometimes pounds), and propane generators tend to consume slightly more volume per hour than diesel equivalents. If your supplier quotes tank capacity in pounds, divide by 4.23 to convert to liquid gallons of propane.
Planning for real outages
Most residential standby generators run at 30 to 60 percent of rated capacity because they are sized to cover the whole house but not everything runs at once. At those load levels a generator is noticeably more efficient than at full throttle, and runtime is significantly longer than the worst-case figure sometimes quoted on the unit’s nameplate.
For critical applications — hospitals, data centers, emergency sites — always hold back at least 20 percent of your calculated runtime as a reserve, because cold starts, fuel temperature effects, and engine age can each add 5 to 15 percent to real-world consumption.
Tips
- Run standby loads at 30 to 60 percent for the best efficiency and runtime.
- Keep a fuel reserve — cold, altitude, and engine age raise consumption.
- Use the data sheet’s own units for both fuel quantity and consumption.
- If your generator has an eco-throttle or economy mode, it adjusts engine speed to match load and can meaningfully extend runtime beyond the rated full-speed figures.