Whole House Generator Size Calculator
Size a standby generator in kW.
Special
Generator Runtime Calculator
Find out how many hours or days a portable or standby generator can run on the fuel you have, how much it burns per hour at different loads, and what a day of backup power costs in fuel.
How it works
- Choose the fuel and enter the generator’s rated output in kW for that fuel.
- Enter the average load as a percentage of the rating — 50% is a common planning figure.
- Enter the tank size and how full it is (large propane tanks hold about 80% of their size).
- Optionally switch to the manufacturer’s published fuel consumption and add a fuel price.
Formula
Fuel per hour = rated kW × full-load burn per kW × (0.15 + 0.85 × load) Usable fuel = tank size × fill % Runtime (h) = usable fuel ÷ fuel per hour
Full-load burn rates used: gasoline 0.15 gal/kWh, propane 0.164 gal/kWh, diesel 0.08 gal/kWh and natural gas 14.5 ft³/kWh, calibrated to typical manufacturer data (for example, a 22 kW standby unit uses about 3.6 gal/h of propane at full load and 2 gal/h at half load). Generators burn fuel even when lightly loaded, which the 0.15 idle share represents. A pound of propane is 1/4.24 gallon; a therm of natural gas is about 96 ft³.
Example
A 7,500-watt portable gasoline generator with a 6-gallon tank runs a refrigerator, freezer, lights and a window AC — about 40% of its rating on average. Gasoline costs $3.40 a gallon.
Inputs
- Fuel
- Gasoline
- Rated output
- 7.5 kW
- Average load
- 40 %
- Tank size
- 6
- Tank unit
- Gallons
- Fuel in the tank
- 100 %
- Fuel consumption
- Estimate from load
- Fuel price (optional)
- $3.4 / gal or therm
Result
Estimated runtime10.9 hoursAt 40% load (3 kW)
Fuel per hour0.55 gal
Fuel per day13.2 gal
Usable fuel6 gal
Planning fuel for an outage
A standby generator rarely runs at full load for long. Refrigerators and furnaces cycle, and the air conditioner is off overnight, so a whole-day average of 30–50% is common. Estimate at 50% for planning and check the 75% row if you rely on heavy loads during heat waves.
For propane, remember that very cold weather reduces how fast a small tank can vaporize fuel. Large standby units are usually paired with 250–500 gallon tanks; your propane supplier can confirm the tank is large enough for the generator’s demand.
Frequently asked questions
How long will a 500-gallon propane tank run a generator?
A 500-gallon tank filled to 80% holds 400 gallons. A 22 kW standby generator at half load burns roughly 2 gallons an hour, so about 190–200 hours — eight days or more of continuous running.
How long will a generator run on a 20 lb propane tank?
A 20 lb cylinder holds about 4.7 gallons. A 5–7 kW dual-fuel portable at half load typically runs about 7–10 hours on it.
Does a generator use less fuel at lower load?
Yes, but not proportionally. Fixed-speed engines burn a noticeable amount of fuel even at light load, so running at 25% uses roughly a third of the full-load fuel rate, not a quarter.
How much natural gas does a standby generator use?
Roughly 14–15 cubic feet per hour per kW of rating at full load — around 300 ft³/h for a 22 kW unit. The gas meter and piping must be able to supply that flow.