Field calculator
Generator runtime calculator (fuel hours)
Planning a generator for an outage or a standby duty starts with how long it will run on the fuel you have. The runtime in hours equals the usable fuel in gallons divided by the fuel consumption in gallons per hour. Enter the tank capacity, the burn rate, and a reserve percentage to hold back so you are not running the tank dry. The number that moves the most is the burn rate, because fuel consumption tracks the electrical load: a diesel generator burns roughly 0.05 to 0.08 gallons per hour per kW near full load and proportionally less when lightly loaded, so use the consumption at your actual load from the manufacturer's data rather than a single fixed figure. For an extended outage, plan a fuel reserve, a refueling contract, and fuel maintenance, since stored diesel degrades over time and water and microbial growth foul it, which is also why standby tanks get periodic polishing. Confirm the runtime against the manufacturer's load-versus-consumption curve.
Result
Generator runtime: hours = usable fuel in gallons / the fuel consumption in gallons per hour. Enter the tank capacity, the burn rate, and a reserve percentage to hold back. The catch is that fuel burn tracks the LOAD: a diesel generator burns roughly 0.05 to 0.08 gallons per hour per kW near full load and proportionally less when lightly loaded, so pull the consumption at your actual load from the manufacturer rather than assuming a fixed rate. For extended outages, plan a fuel reserve, a refueling contract, and fuel polishing or stabilizer, since stored diesel degrades and water/microbial growth fouls it.
anvilfield.com/calculators/generator-runtime-fuel-calculator · Free field calculators and FieldOS. A planning estimate, verify against the code, the manufacturer, and the engineer of record.
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Generator runtime FAQ
What size standby generator do I need?
Size it to the load it backs up from a load calculation, then check it against the worst motor starting step, because inrush can run 2.5 to 3.5 times a motor's rated kVA and stall an engine sized only for running load. Derate for altitude and ambient, and confirm against the maker's curves.
What is the difference between an emergency and an optional standby system?
An emergency system under NEC Article 700 backs up life-safety loads and commonly must restore power within 10 seconds, with independent wiring and listed equipment. An optional standby system under Article 702 protects only property and business, has no code transfer-time limit, and the owner sets the requirements. The classification drives the whole install.
What is a service-rated transfer switch?
A service-entrance-rated transfer switch has the service disconnecting means and the service bonding built into its enclosure, so it acts as the building service disconnect and needs no separate main ahead of it. A non-service-rated switch sits downstream of a separate service disconnect. The service rating decides where the switch lives in the one-line.
Does a generator need a switched neutral?
Only when the transfer switch switches the neutral. A 4-pole switched-neutral switch makes the generator a separately derived system that needs a system bonding jumper at the set per NEC 250.30. A 3-pole switch leaves the neutral solid, so the generator is non-separately derived and gets no bond at the set.
How long can a standby generator run on its fuel?
Diesel runtime depends on stored fuel: a base tank gives roughly 24 hours at full load, an extended tank around 72, and bulk storage with a day tank runs weeks. NFPA 110 sets a minimum by the system Class. Natural gas runs as long as the pipeline holds pressure, which a regional event can curtail.