Quick answer: most 120 V welders need a 3,000–5,000 W generator, and most 240 V welders need 7,000–12,000 W, measured in running (continuous) watts, not peak. For an exact figure, enter your welder’s input amps into our welder generator size calculator.
Getting this wrong is frustrating: the arc sputters, the generator bogs down, or its breaker trips mid-bead. Here’s how to size it properly.
Generator size chart for welders
Typical sizes for modern inverter welders running near full output, plus a 20% margin for arc starts:
| Welder | Welding at | Generator (running watts) |
|---|---|---|
| 120 V flux-core / MIG | 90 A | 3,000 W |
| 120 V inverter stick | 80 A | 3,000 W |
| 120 V inverter stick | 100 A | 4,000 W |
| 120 V MIG | 120 A | 4,000 W |
| 120 V MIG | 140 A | 5,000 W |
| 240 V inverter stick | 125 A | 5,000 W |
| 240 V inverter TIG | 200 A | 6,000 W |
| 240 V inverter stick | 160 A | 7,000 W |
| 240 V inverter MIG | 200 A | 8,000 W |
| 240 V inverter stick | 200 A | 9,000 W |
| 240 V inverter stick | 250 A | 12,000 W |
| 240 V transformer “buzz box” (e.g. AC-225) | 125 A | 9,500 W |
| 240 V transformer “buzz box” | 225 A (full) | about 16,000 W |
If you plan to run a grinder or lights at the same time, add their watts on top. These are estimates from standard arc-voltage formulas (the same ones our calculator uses), and your welder’s own rating plate always wins.
How to size a generator for your welder (step by step)
- Find the input current on the rating plate or in the manual. It’s usually marked I₁max (maximum input amps) and sometimes I₁eff (effective input amps, averaged over the duty cycle). Use I₁max for sizing.
- Note the input voltage: 120 V, or 208/230/240 V.
- Multiply volts × amps. That’s roughly the power (VA) the welder draws at full output. For example, 120 V × 20 A = 2,400 VA.
- Add a margin for arc starts: about 20% for inverter welders and 30% for transformer welders, which surge harder. 2,400 × 1.2 = 2,880 W.
- Add anything else you’ll run at the same time: an angle grinder (about 1,000 W), work lights, a fan.
- Round up to the next generator size, using its running watts.
- Check the outlet. A 240 V welder needs a 240 V receptacle on the generator that matches its plug.
Or let the calculator do steps 3–6. It also estimates the draw from your welding amps if you can’t find the rating plate.
Running watts vs peak watts
Generators list two numbers:
- Running (continuous or rated) watts: what it can supply all day.
- Peak (starting or surge) watts: what it can deliver for a few seconds.
Always size to running watts. A welding arc is a continuous load, so a generator rated “5,000 W peak / 4,000 W running” is a 4,000 W generator for welding purposes.
Inverter welder or transformer welder?
Inverter welders (most machines sold today, which are light and compact) are far more efficient. They draw less current for the same welding output, so they run on a much smaller generator.
Transformer welders (heavy, older machines like the classic Lincoln AC-225 “tombstone”) have poor power factor and draw much more current. Running one at full output takes a very large generator, often more than the welder is worth. Turning the amps down helps a lot: an AC-225 at 125 A for 1/8” rod needs about 9,500 W, compared with roughly 16,000 W at its full 225 A.
Does the type of generator matter?
Yes, especially for inverter welders:
- Inverter generators produce very clean power (low THD, total harmonic distortion), which modern inverter welders and electronics prefer. They’re also quieter and more fuel-efficient, but they cost more per watt and are less common in large sizes.
- Conventional (open-frame) generators give more watts for the money. Look for one rated under about 5% THD if you’ll run an inverter welder. Some cheap generators have high distortion and can make inverter welders cut out or behave erratically.
- 240 V outlets: larger generators have twist-lock L14-30 or 14-50 outlets. Many 240 V welders use a 6-50 plug, so you may need a properly rated adapter or a new cord end, wired correctly.
- Dual-fuel generators run on gasoline or propane. On propane they usually produce about 10% less power, so size up if you’ll run on LP.
Will my generator run my welder?
Will a 3,000 W generator run a welder?
Yes, a small one. A 3,000 W (running) generator handles a 120 V inverter flux-core or MIG welder at about 90 A, or an inverter stick welder at about 80 A. That’s enough for 1/16”–3/32” rods and thin to medium steel. It won’t run a 240 V welder at any useful setting.
Will a 3,500 or 4,000 W generator run a welder?
A 4,000 W generator runs most 120 V inverter stick welders at around 100 A (1/8” 6013 or 7018) and 120 V MIG welders at about 120 A. A 3,500 W generator falls in between: fine for flux-core and light stick work.
Will a 5,000 W generator run a welder?
Yes. It covers a 120 V MIG welder at its full 140 A, or a 240 V inverter stick welder at about 125 A. It’s a good all-round size for home and farm welding with a small machine.
What can a 7,500 W generator run?
A 7,500 W generator runs a 240 V inverter stick welder at about 160 A or a 240 V MIG welder at a little under 200 A, with some power left over. For welding, it’s one of the most useful sizes for a home shop that uses a 240 V multi-process machine.
Will a 9,500 W generator run a 260 A welder?
Only at reduced amps. A 260 A inverter welder at full output needs about 12,000 W, but on a 9,500 W generator it runs comfortably up to about 200 A stick, which covers 1/8” and most 5/32” rod work. If you need the full 260 A regularly, consider a dedicated welder generator instead.
What size generator for a Lincoln AC-225 (buzz box)?
At full 225 A, about 15,000–16,000 W. At typical settings for 1/8” rod (90–125 A), about 8,000–9,500 W. Its rating plate lists about 50 A input at 230 V at full output. If you need to weld away from power often, a modern inverter stick welder on a 5,000–7,000 W generator is usually the cheaper route.
What size generator for a 140 amp welder?
For a 120 V 140 A MIG welder at full output, about 5,000 W. At the 90–120 A settings most people use for 1/8” steel, 3,500–4,000 W is usually enough. Check the rating plate’s I₁max to be sure.
Other things that affect generator size
- Altitude and heat: generators lose power in thin or hot air, roughly 3–4% per 1,000 ft (300 m) above sea level on many engines. Check the generator’s manual for its derating.
- Long welds: running a generator at 90–100% of its rating for hours shortens its life and makes it run hot. Leave headroom if you weld all day.
- Extension cords: use short, heavy-gauge cords rated for the current. A long thin cord drops voltage and makes the arc unstable. It’s better to move the generator closer and run longer welding leads instead.
- Other loads: a compressor, a plasma cutter or a big grinder can each add 1,500–3,000 W or more.
Safety
- Never run a generator indoors, in a garage, or near windows and doors. Carbon monoxide is invisible and kills.
- Follow the generator manual’s grounding instructions for your setup.
- Don’t backfeed a house through an outlet. Use a properly installed transfer switch.
- Let it cool before refuelling.
When a welder generator makes more sense
If you weld away from mains power often, or you need more than about 200 A, a dedicated engine-driven welder generator is usually better value than a huge standby generator plus a welder. It’s built for welding duty cycles and also gives you 4,000–10,000 W of power for tools. See our guide to the best welder generator combos.
FAQ
How many watts does a welder use?
Small 120 V welders use about 2,000–4,000 W near full output. 240 V welders use about 4,000–10,000 W, and big transformer machines even more. The welder only draws that much while the arc is lit.
How many watts does a 90 amp welder use?
About 2,000–2,500 W for a modern 120 V inverter flux-core welder at 90 A. With a starting margin, a 3,000 W generator handles it.
How many amps does a welding machine use?
At full output, a 120 V hobby welder typically draws 15–25 A, and a 240 V welder 20–50 A. The exact figure is the I₁max on the rating plate.
How much electricity does a MIG welder use?
Only while you’re welding. A 120 V MIG welder at 120 A uses about 3 kW of power while the arc is on. Because you stop often to reposition and clean, real hourly energy use is usually well under that.
Can I run a welder on an inverter generator?
Yes, if its running watts are high enough. Inverter generators have very clean power, which suits inverter welders. Two smaller inverter generators can often be linked in parallel to run a 120 V welder.
Is it OK to run a welder at less than full power on a smaller generator?
Yes. The welder only draws what the setting needs. A 240 V welder that needs 9,000 W at full output may run fine at 100–130 A on a 5,000–6,000 W generator, as long as the generator has the right 240 V outlet.
How this guide was researched: generator sizes are calculated with the same method as our calculator: IEC 60974-1 arc-voltage load lines plus typical inverter and transformer efficiency, with a 20–30% margin. Your welder’s rating plate and manual always take priority. See how I test and review.