GENERATOR SIZE
GENERATOR SIZE
kW = Σ run watts + largest motor surge
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Running watts + 2.5× surge on largest motor + 20% headroom. Estimate only — verify with a licensed electrician and local code/inspector before purchase or installation. Not a substitute for engineered drawings.
About this calculator
This generator size calculator adds up running wattage for the loads you want to back up, applies a startup surge to the largest motor (refrigerator, well pump, AC compressor — typically 2.5× running), and recommends a portable or standby generator size. Inductive loads — anything with a motor — are the trap that undersized generators fall into: nameplate running watts is fine, but the locked-rotor inrush at startup is 2-4× higher and stalls the genset. The calculator picks a generator with at least 20% headroom on running watts and headroom for the surge. Use the "essentials" preset for a portable, or "whole home" for a standby. ESTIMATE ONLY — interlock kits, transfer switches, and fuel sizing must be verified by a licensed electrician.
Common questions
How many watts to back up the essentials?
A typical "essentials" load — fridge, freezer, furnace blower, lights, phone chargers, modem/router, microwave — totals 3,000–4,500 running watts. Add a well pump and you are at 5,000–6,000 W running with 9,000+ W surge. A 7,500 W portable generator covers most of those scenarios.
What is starting (surge) wattage?
Motors draw 2-4× their running wattage for the half-second they are spinning up. A 700 W fridge surges to ~1,800 W; a ½ HP well pump runs at 1,000 W and surges to 3,000-4,000 W. The generator must have enough surge headroom (sometimes called "peak watts") to swallow the largest single motor without stalling.
Do I need a transfer switch?
Anything wired into the home panel needs a manual transfer switch or a UL-listed interlock kit per NEC 702. Backfeeding through a dryer outlet without an interlock is illegal and can kill a lineman working on the de-energized service. Portable generators powering extension cords directly to appliances are exempt.