Welder Generator Size Calculator (120V & 240V)

A 140A-class 120V wire-feed machine draws about 2,400W of input at full output — with 25% headroom, 3,000W, so the 3,500–4,500W class covers it (and matches the manufacturers’ common advice of a generator ~1.5–2× the welder’s input). You can weld lighter — at 90A output the draw falls well under 2,000W — but sizing for full output means never blaming the generator for a bad bead.

Only on a genuinely large one: a 180–225A stick/MIG machine draws around 7,000W of input, needing roughly 8,750W with headroom PLUS a 240V outlet — a 9,000–12,000W class portable working hard in pulses. It works, and fabricators do it, but at that spend compare an engine-driven welder/generator: purpose-built for the duty, with better arc characteristics and thousands of watts of auxiliary output for grinders and lights.

Modern inverter welders are mostly generator-rated — Miller and Lincoln publish compatibility guidance — but two conditions matter: enough capacity that voltage doesn’t sag at arc strike (the 1.5–2× input rule exists for this), and reasonably clean supply, which inverter GENERATORS provide inherently and open-frame units mostly manage under light distortion. The failure pattern to avoid is a marginal open-frame generator sagging hard at every strike; that stresses the welder’s front-end electronics over time. When the manual specifies a minimum generator size, believe it.

Arc characteristics follow supply stiffness. On a marginal generator, voltage dips at strike and under wire burnback, showing up as harder starts, more spatter and inconsistent penetration — welders read it as "the machine acting up". The same machine on a properly-sized generator (or the grid) behaves normally. Diagnosis is cheap: if beads improve on wall power, the generator is undersized or the cord run is dropping voltage — 10-gauge cord, short run, next size up. Duty-cycle discipline also matters: generators heat-soak on long beads just like welders do.