Emergency Generator Fuel Calculator
Calculate fuel storage needs for emergency generator runtime based on generator size, load, fuel type, and duration.
About this calculator
Running a generator through a multi-day outage means stocking enough fuel in advance — and this calculator works backward from your generator's size and expected duty cycle to tell you exactly how much to buy. It starts from published baseline consumption rates at 50% load: about 0.075 gallons per hour per kilowatt for gasoline, 0.065 for diesel, and 0.10 for propane, reflecting each fuel's differing energy density and engine efficiency. Because fuel burn doesn't stay fixed as load changes, the calculator scales that baseline linearly by your entered load percentage relative to 50% (loadFactor = loadPercent / 50), then multiplies by generator size and your target runtime in hours to get total gallons needed. A safety margin percentage (15% by default) is added on top as a reserve buffer, since real outages routinely run longer than planned and fuel delivery may be disrupted.
The result is also expressed in standard 5-gallon cans, rounded up, plus how many hours of runtime a single can provides at your specified load — useful for sizing a resupply run. Fuel cost uses simple average per-gallon prices for each fuel type, and the tool flags each fuel's typical shelf life (gasoline ~6 months, diesel ~12, propane essentially indefinite) as a reminder that stored gasoline needs periodic rotation or a stabilizer additive to stay usable. Keep in mind these are baseline manufacturer-style figures — actual consumption varies with generator age, altitude, ambient temperature, and how well-tuned the engine is, so treat the total as a solid starting estimate rather than an exact number.
Inputs
Results
Total fuel needed (gal)
46.6
How to Use This Calculator
- Enter Generator Size (kW) and Average Load (%) expected during operation.
- Set Target Runtime (hours) for the emergency event.
- Select Fuel Type (gasoline, diesel, or propane).
- Enter Safety Margin % to ensure adequate reserve.
- Review Total Fuel Needed (gallons) and Consumption Rate (GPH) to plan storage and resupply logistics.
How the result changes with Generator size (kW)
| Generator size (kW) | Total fuel needed (gal) |
|---|---|
| 3.75 | 23.3 |
| 5.63 | 35 |
| 11 | 68.3 |
| 19 | 118 |
What each input means
- Generator size (kW)
- Rated output of the generator in kilowatts (typical home: 5-22 kW).
- Avg load (%)
- Average load as percentage of rated capacity. 50% is typical for intermittent use.
- Target runtime (hours)
- Total hours of generator operation needed (72 hrs = 3-day outage).
- Fuel type (0-2)
- 0 = Gasoline, 1 = Diesel, 2 = Propane (LP).
- Safety margin (%)
- Extra fuel reserve beyond calculated need.
What each result means
- Total fuel needed (gal)
- Total gallons of fuel to store including safety margin.
- Fuel consumption (gal/hr)
- Gallons per hour at specified load.
- 5-gallon cans needed
- Standard fuel cans to purchase and store.
- Runtime per can (hrs)
- Hours of runtime from a single 5-gallon can.
- Actual output (kW)
- Actual power output at your specified load percentage.
- Est. fuel cost ($)
- Approximate fuel cost based on average prices.
- Safety reserve (gal)
- Extra fuel included as safety buffer.
- Fuel shelf life (months)
- Typical shelf life: gasoline ~6 mo, diesel ~12 mo, propane indefinite.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersGenerator size (kW) = 7.5, Avg load (%) = 50, Target runtime (hours) = 72, Fuel type (0-2) = 0 = 5 input(s) provided
- Calculate Total fuel neededTotal fuel needed = baseFuelGallons + safetyReserveGallons46.6 = 46.6
- Calculate Fuel consumptionFuel consumption = generatorKW * baseRate * loadFactor0.56 = 0.56
- Calculate 5-gallon cans needed5-gallon cans needed10 = 10
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does lowering my load percentage below 50% reduce fuel needs so directly?
The calculator applies a loadFactor of loadPercent / 50, so consumption scales linearly around the 50%-load baseline rate for your fuel type — running at 25% load roughly halves consumptionGPH compared to 50%, while running at 100% roughly doubles it. This is why sizing your generator to only power essential circuits, rather than running it near full capacity, meaningfully stretches your stored fuel across a longer outage.
Why does propane have a higher baseline consumption rate than gasoline or diesel?
Propane's baseline rate of 0.10 gal/hr per kW is higher than gasoline's 0.075 or diesel's 0.065 because propane carries less energy per gallon than either liquid fuel, so a generator needs to burn more of it to produce the same kilowatt output. Propane's tradeoff is its essentially indefinite shelf life, versus gasoline's roughly 6-month usable window before it degrades and needs a stabilizer or rotation.
How much extra fuel does the safety margin actually add?
The safety margin is a straight percentage of your calculated base fuel need (safetyReserveGallons = baseFuelGallons × safetyMarginPct / 100), added on top rather than replacing any of the base estimate. At the default 15%, a generator that needs 50 gallons for its target runtime would get an extra 7.5 gallons of reserve, sized to cover the common case of an outage running somewhat longer than initially planned.
Why does the calculator warn about fuel shelf life instead of just reporting a fuel quantity?
Stored fuel isn't indefinitely usable — gasoline is estimated to last about 6 months before it degrades, diesel about 12 months, while propane stored in a sealed tank essentially doesn't expire. If your emergency preparedness plan holds fuel in reserve for longer than these windows without rotating it, the shelfLifeMonths output is a reminder that the gallons you've stored may not run your generator reliably when you actually need them.
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