What size generator do you need to run a welder?
Enter the numbers off the welder's nameplate and see the running watts and kVA it pulls at full output, the inverter rule of thumb, and what changes for a transformer machine.
Updated November 2026
Quick answer
Multiply the input volts by I1max on the plate. A 240 V welder with 30 A I1max pulls 7,200 W (7.2 kVA) at full output, so an inverter needs a generator with at least that many running watts to give you full output. For a 200 A inverter, Weldclass's rule of thumb is 8 kVA minimum and 10 kVA ideal. Transformer machines need more: Lincoln says its AC/DC 225/125 (K1297) needs at least a 15,000 W generator, against 11.5 kVA on the plate. The welder's manual has the last word.
How generator size is worked out
- The calculator's starting plate: 240 V × 30 A = 7,200 W, or 7.2 kVA.
- A published transformer plate, Lincoln's AC/DC 225/125 (K1297): 230 V × 50 A = 11,500 W, or 11.5 kVA.
- Lincoln's own figure for that machine is at least 15,000 W, 30% over the plate number.
The calculator doesn't add a safety margin of its own, because no welder maker publishes one rule for it. It shows the plate number, and for inverters, Weldclass's table.
Weldclass generator sizes for inverter welders
Weldclass, a welder brand, publishes this rule of thumb for inverter machines by their maximum output. It is their table, not a code.
Weldclass says the minimum is the smallest they suggest to keep the risk of voltage spikes down and may not be enough for full output, and the ideal will allow a higher output from the welder. A welder that falls between two rows reads the next bigger row.
Other welder power questions
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Questions welders keep asking about generators
What size generator do I need to run a 200 amp welder?
For an inverter, Weldclass's rule of thumb for 180-200A is 8 kVA minimum and 10 kVA ideal. Check it against the plate: a 240 V welder with 30 A I1max pulls 7,200 W at full output. A transformer machine needs a lot more. Lincoln says its AC/DC 225/125 (K1297), a 225 A AC transformer machine, needs at least a 15,000 W generator.
Can a 7,000 watt generator run a welder?
It depends on the welder, not the amps on the front panel. Multiply input volts by I1max. A 240 V welder with 30 A I1max pulls 7,200 W at full output, so a 7,000 W generator comes up 200 W short and can't give you full output. Turned down, the welder pulls less. A 120 V welder whose plate says 20 A I1max pulls 2,400 W, well inside it.
Does a transformer welder need a bigger generator than its plate says?
It can. Lincoln's AC/DC 225/125 (K1297) pulls 50 A at 230 V at rated output, 11.5 kVA by the plate, and Lincoln says it needs at least a 15,000 W generator. That is 30% over the plate number. When the maker publishes a generator size, go by it.
Watts or kVA: which number do I compare?
Volts times amps gives volt-amps, and dividing by 1000 gives kVA. The calculator shows both: 240 V × 30 A is 7,200 W, or 7.2 kVA. On the generator, compare against the running (continuous) watts, not the starting or peak watts. Weldclass's table is in kVA.
Do inverter welders need a special generator?
They want clean power. Generator makers list it as THD (total harmonic distortion), and lower is cleaner. Check the THD on the generator spec sheet against what the welder manual asks for. Weldclass, whose sizes are on this page, also asks for a low-THD generator.
Where do I find I1max on my welder?
On the nameplate or rating label, usually near the input voltage, marked I1max. Older plates often list rated primary current, or input amps at rated output, instead. Use that. Take the figure for the voltage you will run on, because a dual-voltage machine pulls more amps on 120 V than on 240 V.
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