SettingsSettings for this calculationUS
The sum of the running wattage of everything you'd want powered at once.
Add up the rated running watts (printed on a label or in the manual) of the fridge, well pump, furnace blower, lights, and other items you'd run simultaneously during an outage.
The extra surge (beyond its own running watts) needed to start your single largest motor.
Motors (well pumps, AC compressors, refrigerators) briefly draw 2-3x their running watts to start. Only the single largest one's surge matters, since starts rarely overlap — enter its starting watts minus its own running watts, which is usually already counted in the total above.
Extra headroom so the generator isn't run at its absolute maximum continuously.
Running a generator at 100% capacity continuously shortens its lifespan — 20% headroom is a common recommendation.
Recommended generator size
6,600 W (recommended generator rating)
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
- 3,500 W
- Peak (with largest surge)
- 5,500 W
They open the calculator with your figures already in it
Generator Sizing Calculator: 6,600 W (recommended generator rating) — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- 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
Inputs used
- Total Running Watts
- 3500
- Largest Motor's Additional Starting Surge
- 2000
- Safety Margin (%)
- 20
Intermediate steps
- Total running watts
- 3,500 W
- Peak (with largest surge)
- 5,500 W
Confidence note: 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.
What this calculation does not cover
- A generator carries two ratings - the watts it holds continuously and a higher surge it absorbs for a second or two - and this returns one number without saying which of the two to compare it against. Alternators are rated in kVA at a stated power factor, and a stalled motor's inrush arrives at a very poor one, so clearing a watt total is not the same as clearing the motor-starting column on the same spec sheet.
- Only the largest single surge is counted, on the assumption that two motors never start in the same instant. Nothing here enforces that: a well pump, a septic pump and a condenser that can all call together need all three surges, and after a transfer the whole building restarts at once. Where load-shed or sequencing controls are what make the single-surge assumption true, those controls have become part of the sizing.
- No voltage or frequency dip is calculated. Sets usually fail a motor start by sagging rather than by overloading - contactors drop out and drive buses undervolt while the engine is barely loaded - and how deep a dip a machine rides through depends on its alternator and excitation, which two sets carrying the same badge kW do not share.
- The answer is a required capacity, not a rating you can read straight off a badge. Engine output falls with altitude and intake air temperature, a set rated on propane delivers less on natural gas, and standby, prime and continuous are different ratings of the same machine - compare this figure against the rating that matches your duty and site conditions.
- This is not an electrical load calculation, and it says nothing about the installation. Sizing a dwelling's service by the code's demand-factor rules, the transfer switch rating, and the neutral bonding and backfeed protection that keep a running set off the utility's lines are separate work for a licensed electrician.
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.
Computed in your browser — nothing you enter is uploaded. 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.
Sources checked 2026-09-02 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- 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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