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Generator Sizing Calculator

Estimate the minimum running-watt capacity a backup generator needs for your household loads.

Computed in your browser — nothing you enter is uploaded. Figures are presented in United States units and terminology, and each formula’s basis is cited under regulatory standards below.

Last verified 2026-08-25 · v1.0.0

Market
Imperial · sales tax

Recommended generator size

6,600 W (recommended generator rating)

Check your inputs

This is a rule-of-thumb estimate. For whole-home standby systems, an electrician performs a full load calculation accounting for which circuits can run simultaneously.

Total running watts
3500 W
Peak (with largest surge)
5500 W

With the figures above, the recommended generator size comes to 6600 W (recommended generator rating). The method behind this is well established, though site conditions and material batches will move it somewhat. This is presented for United States. Building in another market? Change the selector above and the units and terminology follow.

Add the equipment this sizes

This result is a specification — 6,600 W (recommended generator rating) — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.

Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Regulatory standards & verification citations

  • Standard sizing rule of thumb: generator capacity >= sum of all running watts running simultaneously, plus the largest single motor's additional starting-surge watts (beyond its own running watts), plus a safety margin

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Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Frequently asked questions

Why only count the largest motor's surge, not every motor's?
Motors briefly surge only for the second or two it takes to start, and it's uncommon for multiple large motors to start at the exact same instant — sizing for the single largest surge on top of everything else running is the standard practical approach.
Should I size for every circuit in my house?
Only if you want true whole-home coverage. Most portable and mid-size standby generators are sized for essential circuits only (fridge, some lights, furnace blower, a few outlets) — decide what you actually need powered during an outage first.
Is a bigger generator always better?
Not necessarily — oversizing costs more upfront and burns more fuel per hour even at partial load. This calculator estimates the minimum recommended size; going moderately above it for headroom is reasonable, but extreme oversizing has diminishing returns.