To run a welder on a generator, you need a generator that produces at least 125% of the welder’s running wattage with acceptable total harmonic distortion (THD). For most 240V shop welders, that means an 8,000-14,000 watt generator. For 120V welders, a 4,000-7,000 watt unit covers it. Inverter generators produce cleaner power but cost more per watt.
Calculating Generator Size for Your Welder
The formula is simple: multiply the welder’s input amperage by its input voltage to get running watts. Then add margin for startup surge.
Running watts = Input amps x Input voltage
Required generator output = Running watts x 1.25 (minimum)
Worked Examples
| Welder Type | Input Voltage | Input Amps | Running Watts | Min. Generator Size |
|---|---|---|---|---|
| Small MIG (Lincoln 140) | 120V | 20A | 2,400W | 3,500W |
| Mid MIG (Hobart 210) | 240V | 28A | 6,720W | 8,500W |
| Stick (Lincoln AC-225) | 240V | 50A | 12,000W | 15,000W |
| TIG (Miller Dynasty 210) | 120/240V | 28A (240V) | 6,720W | 8,500W |
| Small Stick (Lincoln AC/DC 225/125) | 240V | 36A | 8,640W | 11,000W |
These calculations assume the welder is the only load on the generator. If you’re also running a grinder, lights, or other tools, add their wattage to the total before sizing the generator.
Why 125% and Not 100%?
Three reasons. First, welders have variable power draw. Striking an arc creates a momentary surge that exceeds steady-state draw. Second, generators lose output capacity at elevation and high temperatures. A generator rated at 10,000 watts at sea level may only produce 8,500 watts at 5,000 feet elevation. Third, running a generator at 100% load continuously shortens its life and increases fuel consumption disproportionately. Operating at 75-80% load is the sweet spot for longevity and efficiency.
Inverter Generators vs. Conventional Generators
This distinction matters for welding. The two types produce fundamentally different power quality.
Conventional Generators
Conventional generators produce AC power directly from the alternator, regulated by engine speed. They’re cheaper per watt, handle surge loads well, and come in larger sizes. The downside is power quality. THD (total harmonic distortion) on a conventional generator runs 6-25%, depending on the model and load.
Best for: Transformer-based welders (Lincoln AC-225, older MIG machines), stick welding in the field, situations where raw power output matters more than power quality.
Inverter Generators
Inverter generators produce AC power, convert it to DC, then invert it back to clean AC with precise frequency control. THD is typically under 3%. They’re also lighter, quieter, and more fuel-efficient. The downside is cost and peak power limitations. Inverter generators cost roughly double per watt compared to conventional units.
Best for: Modern inverter welders (anything with a digital display or microprocessor controls), TIG welding where arc stability matters, job sites where noise restrictions apply.
| Feature | Conventional Generator | Inverter Generator |
|---|---|---|
| THD | 6-25% | Under 3% |
| Cost per watt | $0.50-$1.00 | $1.00-$2.50 |
| Weight per kW | ~20 lbs/kW | ~12 lbs/kW |
| Noise at 25 feet | 70-80 dB | 55-65 dB |
| Fuel efficiency | Good at full load | Excellent at partial load |
| Surge handling | Excellent (120-150% rated) | Moderate (110-125% rated) |
| Max common size | 15,000W+ | 7,000-9,000W |
Engine-Driven Welders: The Best Option for Field Work
If you weld off-grid regularly, consider an engine-driven welder/generator combo. These units are purpose-built for welding and include auxiliary power outlets for tools. Lincoln, Miller, and ESAB all make them.
Engine-driven welders produce clean welding power independent of engine speed. They also provide 120V and 240V auxiliary power for grinders, lights, and other tools. The welding output is superior to running a standalone welder on a generic generator because the power conversion is optimized for arc welding.
Popular models like the Lincoln Ranger 250 GXT or Miller Bobcat 250 deliver 250+ amps of welding output plus 10,000-11,000 watts of auxiliary power. They’re not cheap ($4,000-$8,000), but they eliminate the power quality headaches of running a welder on a conventional generator.
Setup and Safety
Grounding the Generator
Generators used in the field need proper grounding. Drive a copper ground rod at least 4 feet into the earth and connect it to the generator’s ground terminal with a ground clamp and 8 AWG copper wire. Some jurisdictions and job sites require ground-fault circuit interrupters (GFCIs) on all generator outlets. Requirements vary by jurisdiction.
Fuel and Ventilation
Generators burn gasoline, diesel, or propane and produce carbon monoxide (CO). Never run a generator inside a garage, shop, or enclosed space. CO is odorless and kills quickly. Position the generator at least 20 feet from any building with the exhaust pointed away from doors and windows.
Per OSHA regulations (29 CFR 1926.302 and 1926.353), engine-driven equipment must be placed where exhaust fumes cannot accumulate in occupied spaces. This applies to job sites and home shops alike.
Extension Cord Between Generator and Welder
If the generator can’t sit next to the welder, use the shortest possible extension cord with adequate wire gauge. For a 240V welder, this means a 6 AWG or 8 AWG cord, depending on the amperage draw. See our extension cord for welder guide for detailed gauge and length specifications.
Never daisy-chain multiple extension cords. Each connection point adds resistance and creates a potential failure point.
Common Problems Running Welders on Generators
Erratic arc or wire feed. Dirty power (high THD) from a conventional generator. The welder’s electronics can’t maintain stable output. Switch to an inverter generator or add a power conditioner.
Welder won’t start. Generator output is too low. Check the actual voltage at the outlet with a multimeter under load. Generators produce rated power at full throttle only. Some have eco modes that reduce engine speed and available wattage.
Generator bogs down when welding starts. The generator is undersized. Striking an arc pulls maximum current for a moment. If the generator can’t handle the surge, the engine bogs, voltage drops, and the arc either won’t start or immediately dies.
Digital display errors on the welder. Voltage fluctuations and frequency variations from a conventional generator confuse microprocessor controls. An inverter generator fixes this. Some welders have a “generator mode” that widens the acceptable input voltage range.
Excessive fuel consumption. Running a generator at near-maximum load burns fuel fast. A 10,000-watt conventional generator at full load burns 1.2-1.5 gallons of gasoline per hour. An inverter generator at the same output uses roughly 30% less fuel due to variable engine speed.
Altitude and Temperature Derating
Generators lose power at altitude and in high heat. The engine can’t breathe as efficiently in thin or hot air, reducing output.
| Condition | Power Loss | Mitigation |
|---|---|---|
| Every 1,000 ft above 2,000 ft elevation | ~3.5% per 1,000 ft | Upsize generator or re-jet carburetor |
| Every 10 degrees F above 77 degrees F | ~1% | Shade the generator, ensure airflow |
At 5,000 feet elevation in summer heat, a 10,000-watt generator might produce only 8,000 usable watts. Factor this into your sizing if you weld at elevation.
Generator Sizing Quick Reference
For those who just want the bottom line:
- 120V MIG welder (140-180A output): 4,000-6,000 watt generator
- 240V MIG welder (200-250A output): 8,000-12,000 watt generator
- 120V stick welder: 3,500-5,000 watt generator
- 240V stick welder: 10,000-15,000 watt generator
- TIG welder (200A): 7,000-10,000 watt generator (inverter generator preferred)
- Plasma cutter (40-60A): 5,000-8,000 watt generator
Oversize by at least 25%. Buy the biggest generator your budget allows. You’ll never regret having extra capacity, but you’ll constantly fight an undersized unit.