Generator Runtime Calculator: Estimate Fuel Needed

The basic formula

Generator run time is simply usable fuel divided by burn rate. The formula is: run time (hours) = usable fuel (gallons or pounds) / fuel consumption rate (gallons or pounds per hour). Usable fuel is the tank capacity minus any reserve or unusable volume. For propane, use pounds rather than gallons, since tank ratings are often given in pounds or in gallons of water capacity.

Fuel consumption rate is not a fixed number. It changes with electrical load, engine wear, fuel type, and ambient conditions. The most reliable number is the consumption figure published by the maker at a stated load, usually half load or full load. If the maker gives a figure at quarter load, use that when you expect light use. If they only give one number, assume it is at full load, and expect lower consumption at lighter loads.

Load factor is the key variable

Most portable generators burn considerably less fuel at half load than at full load. If the maker states consumption at full load and you plan to run at half load, your estimate will be too short. The opposite is also true: running near full load burns more fuel than the half-load figure suggests.

To estimate your load factor, add up the running watts of the equipment you will power at the same time, then divide by the generator's rated output. For example, if you need about 1,500 running watts from a 3,000-watt generator, your load factor is roughly 0.5. This is the number to use when comparing published fuel consumption figures. Learn more about the difference between running watts and starting watts.

Worked example with a 4,000-watt generator

Assume a 4,000-watt portable generator has a 4-gallon fuel tank. The maker states a fuel consumption of 0.5 gallons per hour at half load (2,000 watts). At that load, the formula gives: 4 gallons / 0.5 gallons per hour = 8 hours of run time. This is the figure the maker would likely print on the box.

If you instead run the generator at 3,000 watts (75% load), consumption will be higher, perhaps 0.75 gallons per hour. The same 4-gallon tank then gives about 5.3 hours. If you run a light load of 1,000 watts, consumption may drop to 0.3 gallons per hour, giving over 13 hours. These numbers are illustrative; use the maker's consumption figures for your specific model.

• Always use the consumption figure that matches your expected load. Do not mix full-load consumption with a half-load estimate.
• Add a safety margin: if your calculation says 8 hours, plan for about 7 hours of actual use before refueling.
• For propane, remember the tank's usable capacity is not the total weight; a 20-pound tank holds about 4.7 gallons of liquid propane. Weigh the tank before and after use to calibrate your estimate.

Fuel types change the numbers

The energy content of fuel affects how much you burn per hour. Gasoline, propane, and natural gas have different energy densities. A generator that runs 8 hours on 4 gallons of gasoline may run fewer hours on the same tank volume of propane, because propane contains less energy per gallon. This is why dual-fuel generators often list separate run times for gasoline and propane.

For natural gas, the comparison is even more complex because gas is measured in cubic feet, not gallons. Natural gas, as explained by the EIA, is mostly methane, but its energy content varies by source. See our article on generator fuel types for a deeper look, and compare specific models with dual-fuel generator specs.

Estimating fuel needed for an outage

To estimate how much fuel you need for a multi-day outage, first determine how many hours per day you will run the generator. Most people do not run a portable generator 24 hours a day; they run it in blocks, several hours at a time, and then shut it down. Multiply the daily run hours by the fuel consumption rate at your load.

For example, if you plan to run 6 hours per day at a consumption rate of 0.5 gallons per hour, you need about 3 gallons per day. For three days, that is 9 gallons. Always add at least a 20% safety margin, and store fuel safely. Carbon monoxide safety means you should also plan to position the generator outdoors, far from windows and doors, so refueling is done outside.

Why maker numbers differ from real use

Published run times are measured under controlled conditions, usually at a constant load, at sea level, with new fuel and a clean air filter. Your real conditions are rarely identical. Temperature, altitude, fuel quality, and engine condition all affect consumption. The EIA notes that energy content varies by fuel source; the same is true for gasoline blends.

For a more accurate estimate, run your generator at home under your typical load and measure the actual fuel level. Use a dipstick or a fuel gauge. After several hours, note how much fuel you used. Divide the elapsed time by the fuel used to get your real consumption rate. This is the number to use for future planning. See also our guide on generator fuel tank size and run time.

Using the calculator with buying guides

When you compare generators, look at the fuel consumption figure at the load you expect to use, not just the run time on the box. The run time is the result of tank size and consumption, so two generators with the same tank size can have very different run times. Filter by your load requirement: if you need about 3,000 watts, see the best 3,000 to 5,000-watt generators. For a 5,000-watt class, see best 5,001 to 9,000-watt generators.

If you value low fuel consumption over absolute power, an inverter generator often burns less fuel at partial load than a conventional model of the same size. Inverter generators adjust engine speed to the load, so they consume closer to what you actually need. See how inverter generators work for an explanation.

What to pick for your use

Questions

What is the formula for generator run time?

Run time (hours) = usable fuel (gallons or pounds) / fuel consumption rate (gallons or pounds per hour). Use the consumption figure that matches your expected load.

How do I convert propane tank weight to run time?

A 20-pound propane tank holds about 4.7 gallons of liquid propane. Multiply the tank's usable pounds by the generator's consumption in pounds per hour, or convert to gallons and divide by consumption in gallons per hour.

Can I trust the run time on the box?

Usually not exactly. Published run times are measured at a fixed load, often half load, in controlled conditions. Your real consumption depends on your actual load, altitude, temperature, and fuel quality.

How much fuel should I store for a 3-day outage?

Multiply the hours per day you will run the generator by the fuel consumption rate at your load. For 6 hours per day at 0.5 gallons per hour, plan on 3 gallons per day, so about 9 gallons for 3 days. Add at least 20% as a safety margin.