Rex says
Everyday utility math — the kind you'd otherwise pull up four browser tabs for. I keep it to one clean answer.
Try a scenario
Click to load — tweak from there.Inputs
Result
Total fuel cost for outage
$121
Fuel burn rate
0.72
Total fuel needed
34.6
Running load fits? (1=yes)
1
Surge load fits? (1=yes)
1
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How to use this
- 1Enter total running load (W).
- 2Enter largest single startup surge (W).
- 3Enter generator running watt rating (W).
- 4Enter generator surge watt rating (W).
- 5Enter fuel type.
- 6Enter fuel price ($/gal).
- 7Enter expected outage length (hrs).
- 8Read your total fuel cost for outage on the right — it updates as you type.
- 9Hit Share to keep the scenario or send it to someone.
About this calculator
Sizes a portable or standby generator from your selected running and starting loads, then estimates fuel burn and cost for a given outage length. Generators are rated by running watts and a higher surge watts figure; if your total starting load (the largest single motor's startup surge plus everything else already running) exceeds the surge rating, the generator will stall or trip. This tool checks that headroom and converts wattage into gallons of gas, propane, or diesel per hour using typical fuel-consumption curves so you can budget for a multi-day outage instead of guessing at the pump.
Worked example
Using the values the calculator loads with:
Inputs
- Total running load: 5000 W
- Largest single startup surge: 3000 W
- Generator running watt rating: 7500 W
- Generator surge watt rating: 9500 W
- Fuel type: Gasoline (~0.6 gal/hr per 5kW load at 50%)
- Fuel price: 3.5 $/gal
- Expected outage length: 48 hrs
Results
- Total fuel cost for outage: $121
- Fuel burn rate: 0.72
- Total fuel needed: 34.6
- Running load fits? (1=yes): 1
- Surge load fits? (1=yes): 1
What each field means
Inputs
- Total running load (W)
- The total running load used in the calculation, measured in W. Starts at 5000 W so you have a working example on load.
- Largest single startup surge (W)
- The largest single startup surge used in the calculation, measured in W. Starts at 3000 W so you have a working example on load.
- Generator running watt rating (W)
- The generator running watt rating used in the calculation, measured in W. Starts at 7500 W so you have a working example on load.
- Generator surge watt rating (W)
- The generator surge watt rating used in the calculation, measured in W. Starts at 9500 W so you have a working example on load.
- Fuel type
- Pick the option that matches your situation — the maths changes per option. Choices: Gasoline (~0.6 gal/hr per 5kW load at 50%), Propane (~1.2 gal/hr equivalent), Diesel (~0.45 gal/hr per 5kW load).
- Fuel price ($/gal)
- The fuel price used in the calculation, measured in $/gal. Starts at 3.5 $/gal so you have a working example on load.
- Expected outage length (hrs)
- The expected outage length used in the calculation, measured in hrs. Starts at 48 hrs so you have a working example on load.
Results
- Total fuel cost for outage
- Returned as a money amount in US dollars and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Fuel burn rate
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Total fuel needed
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Running load fits? (1=yes)
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Surge load fits? (1=yes)
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
How do I size running watts for my house?
Add up the running wattage of everything you'd run simultaneously: fridge (150W), sump pump (800-1300W), furnace blower (600-1000W), a few circuits of lights and outlets (600W), and a window AC if needed (1000-1500W). A 5,000-7,500W generator covers most partial-home setups; a whole-house standby unit typically runs 16,000-22,000W.
Why is surge rating separate from running rating?
Motors draw 2-3x rated running watts for under a second at startup. A generator with 7,500 running watts might only handle 9,000-9,500 surge watts, so if your well pump surges to 3,000W while your furnace and fridge are already running, you can trip the generator's breaker even though average load looks fine.
Propane vs gasoline for backup power?
Propane stores indefinitely without degrading (gasoline goes stale in 3-6 months without stabilizer), burns cleaner, and is safer to store in bulk, but delivers about 10-15% less energy per gallon so burn rate is higher. Gas is cheaper per gallon and more available during regional outages; propane is better for planned, infrequent backup.
How much fuel should I stock for a multi-day outage?
For a mid-size 7,500W generator at moderate load, expect 15-20 gallons of gas per day. A 5-day outage needs 75-100 gallons — well beyond safe residential gasoline storage limits in most areas, which is a strong argument for a natural gas or larger propane standby unit if you live somewhere with frequent multi-day outages.
Accuracy and limitations
- Estimates assume standard, average conditions — local rules, pricing, and materials vary.
- Results are rounded for readability; add a buffer before ordering, booking, or committing.
- Double-check anything with a real cost attached against a local quote.
Related tools
Battery Backup Runtime Calculator
How long a battery keeps your chosen loads running during an outage.
Generator Transfer Switch Cost & Circuit Planner
Circuit count and switch amperage needed for safe generator hookup.
Whole Home Electrical Load Calculator
Simplified NEC-style load calc to check your panel capacity.
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Generator Sizing & Fuel Cost Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/generator-sizing-fuel
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/generator-sizing-fuel" target="_blank" rel="noopener">Generator Sizing & Fuel Cost Calculator — RevenueLab</a> (2026).</p>
Source: [Generator Sizing & Fuel Cost Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/generator-sizing-fuel) (2026).