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
Recommended generator size (W)
6,500
Total running watts
4,100
Peak starting watts
6,100
Estimated fuel burn (gal/hr)
0.55
Fuel cost for 12 hours
$23.72
Calculated requirement (W)
6,100
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One click, a permanent link with your numbers baked in.
How to use this
- 1Enter refrigerators.
- 2Enter sump or well pumps.
- 3Enter furnace blowers.
- 4Enter lighting load (W).
- 5Enter electronics and networking (W).
- 6Enter window ac units.
- 7Enter microwaves.
- 8Enter fuel cost ($/gal).
- 9Read your recommended generator size (w) on the right — it updates as you type.
- 10Hit Share to keep the scenario or send it to someone.
About this calculator
Motors draw two to three times their running wattage on startup. Sizing to running watts alone is the classic mistake that trips a generator the moment the well pump kicks on.
Worked example
Using the values the calculator loads with:
Inputs
- Refrigerators: 1
- Sump or well pumps: 1
- Furnace blowers: 1
- Lighting load: 400 W
- Electronics and networking: 300 W
- Window AC units: 0
- Microwaves: 1
- Fuel cost: 3.6 $/gal
Results
- Recommended generator size (W): 6,500
- Total running watts: 4,100
- Peak starting watts: 6,100
- Estimated fuel burn (gal/hr): 0.55
- Fuel cost for 12 hours: $23.72
- Calculated requirement (W): 6,100
What each field means
Inputs
- Refrigerators
- The refrigerators used in the calculation. Starts at 1 so you have a working example on load. Accepted range: 0–5.
- Sump or well pumps
- The sump or well pumps used in the calculation. Starts at 1 so you have a working example on load. Accepted range: 0–4.
- Furnace blowers
- The furnace blowers used in the calculation. Starts at 1 so you have a working example on load. Accepted range: 0–3.
- Lighting load (W)
- The lighting load used in the calculation, measured in W. Starts at 400 W so you have a working example on load.
- Electronics and networking (W)
- The electronics and networking used in the calculation, measured in W. Starts at 300 W so you have a working example on load.
- Window AC units
- The window ac units used in the calculation. Starts at 0 so you have a working example on load. Accepted range: 0–4.
- Microwaves
- The microwaves used in the calculation. Starts at 1 so you have a working example on load. Accepted range: 0–3.
- Fuel cost ($/gal)
- The fuel cost used in the calculation, measured in $/gal. Starts at 3.6 $/gal so you have a working example on load. Accepted range: 0.5–12 $/gal.
Results
- Recommended generator size (W)
- Returned as a whole number and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Total running watts
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Peak starting watts
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Estimated fuel burn (gal/hr)
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Fuel cost for 12 hours
- Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Calculated requirement (W)
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
How does the home generator sizing calculator work?
Motors draw two to three times their running wattage on startup. Sizing to running watts alone is the classic mistake that trips a generator the moment the well pump kicks on. The underlying maths is: Required = Sum of running watts + largest starting surge.
What do I need to enter?
8 values: refrigerators, sump or well pumps, furnace blowers, lighting load, electronics and networking, window ac units, microwaves, and fuel cost. Each field starts with a sensible default, so you can change one number at a time and watch the result move.
What does the recommended generator size (w) result mean?
It is returned as a whole number and updates live as you edit the inputs, so you can compare two or three versions of a scenario in a few seconds.
Is this calculator free, and do I need an account?
Yes, it's free, and no account is required. Nothing you type is stored on our servers — the maths runs entirely in your browser.
How accurate is the result?
It applies the standard formula exactly, so the arithmetic is precise. Estimates assume standard, average conditions — local rules, pricing, and materials vary.
Who is this tool for?
It's built for anyone doing everyday planning maths — a quick estimate that would otherwise need four browser tabs and a calculator app.
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
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Generator Sizing Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/generator-sizing
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/generator-sizing" target="_blank" rel="noopener">Generator Sizing Calculator — RevenueLab</a> (2026).</p>
Source: [Generator Sizing Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/generator-sizing) (2026).