How Many Watts Does a Welder Need to Run?

Welders are one of the few home and farm tools where the output amperage printed on the front has almost nothing to do with the watts drawn from the wall. A “140-amp” welder does not pull 140 amps from your outlet, and a “200-amp” machine can draw anywhere from 5,000 to 10,000 watts depending on its design and the process you are running. To size a circuit or a backup power source, you need to translate arc amps into input power.

How welding output converts to input watts

The arc itself runs at relatively low voltage. For stick welding a common approximation is 20 volts plus 0.04 volts per amp, so a 100-amp stick arc runs around 24 volts. For MIG the arc voltage is typically 16 to 22 volts on small machines. Arc power is simply arc amps times arc volts. Then divide by the machine’s efficiency to find input power.

A worked example

A 120-volt MIG welder running 110 amps at about 19 volts produces roughly 2,090 watts at the arc. If the machine is 80 percent efficient, input real power is about 2,600 watts. With a power factor around 0.7, the apparent input is closer to 3,700 volt-amps, which is 31 amps at 120 volts for that moment. That is why many 120-volt welders specify a 20-amp circuit and still trip it at the top of their range.

Inverter welders versus transformer welders

Transformer-based welders are heavy and simple, often 60 to 75 percent efficient with power factors of 0.55 to 0.7. Inverter welders use switching electronics, reach 80 to 90 percent efficiency, and many include power factor correction that brings PF above 0.9. For the same arc, an inverter machine might draw 25 to 40 percent less current from the wall. That difference drives everything from breaker size to generator requirements.

These are rough ranges. The rated input current (often labeled I1max and I1eff) on the data plate is the authoritative figure; check the current spec sheet for your specific welder.

Duty cycle and why it changes average power

Duty cycle is the percentage of a 10-minute period a welder can run at a given output before overheating. A 120-volt MIG might be rated 20 percent at 90 amps, meaning 2 minutes of arc time, 8 minutes resting. That limits average energy use considerably. Peak watts still matter for breaker and generator sizing, but a day of hobby welding might only consume 2 to 5 kWh even on a machine that peaks above 5,000 watts.

I1max versus I1eff

European-style data plates list I1max, the maximum input current, and I1eff, the effective input current accounting for duty cycle. Size breakers using the manufacturer’s recommendation, which usually references I1eff, but size power sources and generators to handle I1max, because the arc does not care about the average.

Process and settings matter

Flux-core wire at the top of a small welder’s range pulls more than solid-wire MIG on thin sheet. Stick welding with 1/8 inch 7018 rod at 120 amps needs noticeably more power than 3/32 inch 6013 at 80 amps. TIG welding aluminum on AC typically draws more than DC steel at the same current because of the cleaning cycle and higher arc voltage. If your welder is dual voltage, running it on 240 volts halves the current at the plug and usually unlocks higher output and duty cycle.

FAQ

Why does my 140-amp welder trip a 15-amp breaker?

At the top of its range it can pull 20 to 25 amps or more from the wall, especially if it is a transformer design. Most 140-amp welders call for a dedicated 20-amp circuit with a short, heavy cord.

Does a welder draw power when it is on but not welding?

Very little. Inverter welders idle at roughly 20 to 80 watts to run fans and electronics. Some transformer machines hum at a bit more, but idle draw is small compared to arc draw.

Bottom line

Small 120-volt welders draw roughly 1,800 to 3,000 watts at typical settings, while 240-volt MIG, TIG and stick machines range from about 4,000 to over 10,000 watts at full output. Inverter welders need considerably less than transformer machines for the same arc. Read the I1max figure on your data plate to know your real peak.