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INDEPENDENT GUIDANCE FOR HOME BACKUP POWERHow we evaluate generators

FUEL PLANNING TOOL

Generator runtime calculator

Generator runtime equals usable fuel divided by consumption per hour. Enter your actual inventory, reserve and the consumption figure for the model and load you want to evaluate.

ESTIMATED FUEL DURATION

40 hours

(100 − 20) gallons ÷ 2 gallons/hour = 40 hours

This is fuel-duration arithmetic, not a safe uninterrupted engine runtime or a fuel-system design.

Use the right inputs

  • Enter actual available fuel, not tank nameplate capacity.
  • Subtract a reserve and account separately for other appliances using the fuel.
  • Use the documented consumption at the load you want to evaluate. Changing load changes consumption.
  • For propane, liquid gallons and vapor cubic feet are different units. This tool does not convert between them.

Worked example

With 100 gallons available, a 20-gallon reserve and consumption of 2 gallons per hour, the estimated duration is (100 − 20) ÷ 2 = 40 hours. These inputs are illustrative, not a specification for a particular generator.

What this estimate leaves out

The calculation assumes constant consumption and no refill. It does not validate vaporization, pressure, supply-line capacity, reserve suitability or maintenance intervals. It does not estimate natural-gas utility uptime or battery runtime.

Consult the exact model’s consumption table, such as the Generac 7327 manufacturer reference, and the relevant owner’s manual.

Complete your fuel plan

Propane tank sizing · Fuel running cost · Extended-outage service limits · Propane generator guide

Why does the result change with electrical load?

Fuel consumption changes with the load on the generator. Use the manufacturer’s value for the scenario you are evaluating; a half-load rate does not establish consumption at full load. Run separate scenarios when the load changes, rather than scaling one figure in proportion to output.

Does this estimate apply to natural gas?

This calculator describes a finite stored-fuel inventory. A utility natural-gas connection has no tank inventory to divide by a burn rate. Its backup availability depends on the supply remaining available and the generator’s operating and service requirements.

What should you record with a runtime estimate?

Record the model, fuel unit, load assumption, consumption source and reserve alongside the result. For propane, confirm usable inventory and fuel-delivery capacity with the supplier. See the propane tank planning guide before translating tank size into an outage plan.