Electrical
Generator Size Calculator
Size a generator from running load plus the surge that motor-driven equipment demands at start-up.
Result
Electrical work is life-safety work governed by the NEC or your local code, and must be designed and installed by a licensed electrician. Conductor and overcurrent sizing depends on ambient temperature, conductor bundling, termination ratings, continuous-load rules and more. This is an educational estimate and must never be used as the basis for an installation. Only the largest motor's surge is counted, since simultaneous starts are unlikely but not impossible. Standby generators also need a transfer switch, and back-feeding a panel without one is illegal and can kill utility workers.
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About this calculator
What size generator runs a house?
A 20–22 kW standby unit covers most whole-home needs including central air. Essentials-only backup (fridge, furnace, lights, well pump) often fits in 7–10 kW.
Why do motors need so much starting power?
An induction motor draws several times its running current for the first moment as it overcomes inertia. A generator that only matches running load will stall when the compressor kicks in.
The formula
starting watts = running watts + (largest motor × (surge multiple − 1))
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