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Electrical & Wiring

Whole House Generator Size Calculator

Inputs

100050000
030000

Smart analysis

Recommendations

Tailored to your live inputs and result—guidance only, not professional advice.

  • Interpret

    Treat this as a planning estimate

    Primary result: Generator size = 12.0 kW. Re-run with optimistic and pessimistic inputs to understand the range—not just a single point.

Scenario studio

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Scenario A

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Scenario B

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Plain-English guide

Terms & how it works

Confused by a field? Read the short definitions here while you use the tool.

  • Running watts

    The “Running watts” value used in this tool’s formula. Adjust it to see results update live.

  • Starting surge watts

    The “Starting surge watts” value used in this tool’s formula. Adjust it to see results update live.

Generator kW ≈ (running watts + surge allowance) ÷ 1,000.

Estimate generator kW from essential load and starting surge. Planning estimates only — verify measurements and local codes on site.

How this calculator works

Estimate generator kW from essential load and starting surge. Planning estimates only — verify measurements and local codes on site. Generator kW ≈ (running watts + surge allowance) ÷ 1,000. Scenario: 8,000 W running + 4,000 W surge → ~12 kW generator class.

Generator kW ≈ (running watts + surge allowance) ÷ 1,000.

Scenario: 8,000 W running + 4,000 W surge → ~12 kW generator class.

When to use this Generator Sizing

Common use cases

  • Estimate generator kW from essential load and starting surge.
  • Sanity-check with this scenario: 8,000 W running + 4,000 W surge → ~12 kW generator class.
  • The relationship is: Generator kW ≈ (running watts + surge allowance) ÷ 1,000.

Localized examples

  • Electrical & Wiring: the example on this page uses the same formula as the widget.

What you get

  • Fields: Running watts, Starting surge watts
  • Generator kW ≈ (running watts + surge allowance) ÷ 1,000.
  • Scenario on the page: 8,000 W running + 4,000 W surge → ~12 kW generator class.

How to Calculate Generator Sizing

  1. 1Enter Running watts, Starting surge watts. Use the values you have, not a catalog average, unless that is all you know.
  2. 2Generator kW ≈ (running watts + surge allowance) ÷ 1,000.
  3. 3Scenario: 8,000 W running + 4,000 W surge → ~12 kW generator class.
  4. 4Buy materials after you walk the job. This is a first-pass quantity, not a purchase order.

Governing formula

Generator kW ≈ (running watts + surge allowance) ÷ 1,000.

8,000 W running + 4,000 W surge → ~12 kW generator class.

8,000 W running + 4,000 W surge → ~12 kW generator class.

Variable definitions

  • runningWatts

    Running watts

    The “Running watts” field. Default 8000; typical range 1000–50000.

  • surgeWatts

    Starting surge watts

    The “Starting surge watts” field. Default 4000; typical range 0–30000.

  • The takeoff (Generator kW ≈ (running watts + surge allowance) ÷ 1,000.) ignores waste, code, and what you find when you open the wall. Measure twice on site.
  • Nothing here is uploaded for the calculation itself.

Frequently Asked Questions

Generator kW ≈ (running watts + surge allowance) ÷ 1,000.

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Community

Reviews & Ratings

See what others think about this tool, then leave your own rating to help improve CalculioHub.

4.8

out of 5.0

4 reviews

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Your rating

Aisha K.

Mar 12, 2026

Clean layout and the results update instantly. Exactly what I needed for a quick planning check.

Daniel R.

Feb 28, 2026

Very usable on mobile. Would love a save/export option later, but the math feels solid.

Priya S.

Feb 3, 2026

The FAQ section answered my questions before I even had to search. Smooth dark mode too.

Marcus L.

Jan 19, 2026

Simple inputs, clear outputs. This replaced three bookmarks I used to juggle.

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