Calculator
Estimate how long a propane or diesel generator may run on a given fuel supply — and understand the different situation for utility natural gas.
Fuel Type
Common sizes: 100-gal cylinder, 250-gal, 500-gal, 1,000-gal tank.
20 kW at 50% load
LP tanks are filled to a maximum of 80% capacity per NFPA 58, reserving 20% as vapor headspace for safe thermal expansion. Runtime is based on the 80% usable volume, not the tank's nominal rating.
Estimate only. Fuel consumption varies with load profile, ambient temperature, altitude, engine age, and maintenance condition. These figures are based on typical residential standby generator spec sheets and are intended for planning purposes only. Consult your generator's specification sheet for precise data.
Natural gas is delivered continuously through the utility distribution system — there is no tank, and therefore no finite fuel supply to calculate runtime from. A natural gas standby generator can operate indefinitely as long as the utility gas supply remains active during an outage. Natural gas supply interruptions during major storms are possible but less common than electrical grid failures. The calculator notes this when natural gas is selected and directs you to use the Fuel Cost Calculator for consumption estimates.
Propane consumption depends on generator size and the electrical load fraction. The calculator uses a linear model fitted to published residential generator specifications: consumption rises with load but not proportionally, because the engine drives fixed ancillary systems regardless of output. Enter your generator's rated kW and the expected load percentage for an hourly consumption estimate. Your generator's owner's manual may also list consumption at 25%, 50%, and 100% load for reference.
Lower electrical load reduces fuel consumption per hour and therefore extends runtime. However, generators do not scale fuel use down proportionally with load — they continue powering cooling fans, lubrication systems, and control electronics regardless of electrical demand. The calculator models this with a load slider: reducing load from 100% to 50% extends runtime, but does not double it. Running at very light load (far below the generator's rated capacity) also has diminishing efficiency returns.
Runtime depends on tank capacity, generator size, and load. Enter your specific tank volume (in gallons), generator kW rating, and expected load fraction into the calculator for an estimate. Tank size varies by installation — common residential configurations range from 100 to 1,000 gallons. Actual consumption also varies by model and ambient conditions, so treat the result as a planning estimate rather than a precise measurement.
It depends on tank capacity and generator size and load. Enter your diesel tank volume and generator rating into the calculator for an estimate. Diesel tank sizes for residential standby installations vary from small sub-base tanks to larger external tanks; confirm the usable capacity with your installer. Diesel fuel also has a practical shelf life of 6–12 months without additives, so stored fuel should be rotated or treated before standby use.
Yes. Keeping your propane tank as full as practical before anticipated outage season maximizes available runtime. Propane deliveries during and immediately after major storms can be delayed by road closures and high regional demand. Many propane suppliers offer automatic fill programs that maintain tank levels above a set threshold — that option removes the need to track levels manually. Contact your supplier before storm season to confirm delivery scheduling.
