How it works
If you know the input amps: the welder’s rating plate or manual lists its maximum input current (often marked I1max). Input volts × input amps gives the power it draws. We then add a margin for the surge when you strike an arc: 20% for inverter welders, 30% for transformer welders. Finally we add any other tools you’ll run at the same time, and round up to the next 500 W.
If you only know your welding amps: we estimate the arc voltage from the standard IEC 60974-1 load lines (stick: 20 V + 0.04 × amps; MIG: 14 V + 0.05 × amps; TIG: 10 V + 0.04 × amps). That gives the output power, which we divide by typical efficiency and power factor: about 85% and 0.9 for inverters, 75% and 0.6 for transformer machines.
| Typical welder | Input | Generator (continuous) |
|---|---|---|
| 120 V inverter stick, 90–100 A | 15–20 A | 3,000–4,000 W |
| 120 V flux-core / MIG, 125–140 A | 20 A | 3,000–4,000 W |
| 240 V inverter MIG / stick, 200 A | 25–35 A | 7,000–9,500 W |
| 240 V transformer “buzz box”, 225 A AC | ~50 A | 12,000 W or more |
These are rough ranges. Use the calculator with your welder’s own numbers.
Before you buy a generator
- Match the outlet. A 240 V welder needs a 240 V receptacle that fits its plug (for example NEMA 6-50 or 14-50).
- Prefer clean power. Inverter generators, or models rated under about 5% THD, keep inverter welders happy.
- Welding all day? A generator running near its limit for hours wears fast. Size up, or look at a dedicated welder generator.
- Safety: never run a generator indoors or in a garage. Carbon monoxide is deadly.
FAQ
What size generator do I need for a 120 V welder?
Most 120 V inverter welders draw 15–20 A at full output, so they need about 2,500–3,000 W of continuous generator power. Add more for anything else you plug in.
What size generator for a 240 V welder?
A 240 V inverter welder running 160–200 A typically needs 7,000–9,000 W continuous, and the generator must have a 240 V outlet that matches the welder’s plug. Older transformer welders can need 10,000 W or more.
Should I use peak or running watts?
Use running (continuous) watts. Peak watts only last a few seconds and won’t hold a welding arc.
Can an inverter welder damage a generator?
Usually not, but a generator with high harmonic distortion can upset some inverter welders. Inverter generators, or generators rated under about 5% THD, are the safest choice.
More detail: What size generator do I need for a welder?