What Size Generator for Welding? • WelditU

What Size Generator for Welding?

Sometimes it’s just more practical to tackle welding jobs in the field instead of the convenience of your shop.

Today’s generators and welders are more powerful and portable, making it easier than ever to weld at remote locations, but what size generator for welding is right for you?

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What Size Generator Article Cover

How to Size a Generator for Welding

You’ll find generator output usually expressed in watts. Wattage equals amperage times voltage, meaning a generator rated at 4800 watts can provide 40 amps at 120V, and possibly 20 amps at 240 volts.

Convert output rated in kVa (kilovolt-ampere) to watts by multiplying by 800. For example, a generator rated at 9 kVa multiplied by 800 converts to 7200 watts.

Look for two output ratings for a generator. One for a steady ongoing load known as the operating, or running watts.

“With less than 3% THD, this quiet inverter generator can safely power any 120V or small 240V welders.”

Most generators can also handle a brief output spike that occurs when a motor (or a welder) starts. Typically 25-30% higher than the running figure, this is the start-up or surge watt rating.

Most generators include some wattage value in their model name. You’ll want to check if it represents running or surge watts, as this varies by manufacturer.

Sizing a generator to power a welder starts with knowing the maximum watts the welder will draw. Then you’ll know the minimum watts your welder needs to operate at full capability.

How Many Watts Does Your Welder Use?

It’s rare to find total watts listed for welders, but a few manufacturers provide helpful minimum generator size recommendations.

Everlast PowerARC 140STi owner’s manual

While not all manufacturers bother to provide this information, you can usually calculate watts using voltage and amperage figures from the owner’s manual or the data plate on the welder.

Use the Manufacturer’s Voltage Rating

While you’ll find generators rated for 120 and/or 240 volts, some manufacturers rate their welders at 110, 115, or 230 volts.

For accuracy, it’s best to use the manufacturer’s number listed as U₁ on the data plate. This is the voltage they used to rate the machine and measure amperage values.

Data plate information used to determine what size generator for welding.

Get the Right Amperage Value

Depending on the welder, you may find a handful of different amp ratings, or just one.

Look for the I1max number. It’s the best rating to use because it represents the maximum rated supply current. You may see this referred to as the maximum “inrush” or “surge” current draw on startup.

I₁ₘₐₓ number for welder to calculate maximum watts for generator sizing.
Everlast PowerARC 140STi owner’s manual

Multiply the I1max amperage by the manufacturer’s voltage rating to get the maximum watts required for your welding machine.

Using the above welder, 24.2 amps multiplied by 240 volts gives 5808 maximum watts. The manufacturer, Everlast, recommends a minimum 6000-watt surge generator in the 140 STi owner’s manual.

“The affordable Westinghouse generator is a good size for 120V and small 240V transformer welders. The more powerful model should handle any 240V transformer welder up to 250 amps.”

The I1eff amp number is a thermal rating taking into account rated duty cycle limits (idle time) and heat buildup to size dedicated building circuits. Without adjustment, it’s too low to calculate maximum watts..

When manufacturers give both ratings, I find the I1max is normally 1.7 to 2.2 times greater than the I1eff rating.

Some data plates show just an I1 or “recommended circuit breaker” information, as on a Lincoln 140.

Using 20 amps times 120 volts gives us 2400 watts, but Lincoln recommends a 3000 watt generator for this welder. 3000 watts divided by 120 volts equals 25 amps, which is likely closer to the I1max rating for this machine.

The recommended circuit breaker size may be too small to calculate full operating wattage. A properly operating circuit breaker is designed with a time delay and probably wouldn’t trip because of a brief 25 amp surge.

But this would cause a problem on a generator with a surge rating of 2400 watts.

Finally, don’t make the mistake of using output amperage figures such as I2 or duty cycle amperage.

Adjustments to Consider

Before deciding on what size generator to run your welder, here are a few more things to consider.

Generator altitude adjustment.

Derate for High Altitude Operation

Thinning of air means less power at greater altitudes.

According to one generator manufacturer:

“…air density decreases as altitude increases, causing rated genset engine power to decrease—approximately 3.5 percent each increase of 1000 feet (305 m) (Table 3). It may be necessary to run fewer appliances at higher altitudes.”

Cummins Onan – Generator Operator’s Manual

These Onan generators include an altitude adjustment control. High-altitude kits are available for many generator models.

Cummins Onan – Generator Operator’s Manual

Don’t Forget Accessory Equipment

You may need to operate lights, fans, and air compressors while welding, so add their wattage requirements to your total.

Grinders and chop saws can use up to 1800 watts each. This isn’t an issue if you’re working solo. But in a team situation, sizing the generator for twice the welder’s maximum watts will minimize the impact on your weld of other tools starting up.

“Dirty” vs. Clean Power Generators for Welders

Welder internal power sources fall into two distinct design categories, each handling incoming current in different ways to create weldable power output.

Traditional Transformer-Based Welders

With large transformers made from copper and aluminum, these heavy power sources change AC input current into low voltage/high amperage DC power for welding.

Very reliable and not sensitive to dirty power, traditional welders work well with any generator.

Shows internal view of Hobart 140 MIG welder (cover removed).
Transformer-based welder
Image of welding terms guide linking to free download.

Inverter-Style Welders

With technology to create efficient welding power with much smaller transformers, inverter welders often weigh less than half of traditional welders. They achieve a very stable output thanks to capacitors that store high voltages.

But these sophisticated electronics need quality input power.

Many generators create excessive voltage and frequency fluctuations known as “dirty power”. It’s measured as a percentage of Total Harmonic Distortion, or THD. Dirty power can quickly destroy sensitive electronics or shorten service life with cumulative damage.

Particularly sensitive are older inverter welders using metal-oxide semiconductor field-effect transistor (MOSFET) technology. 

Newer machines based on insulated-gate bipolar transistors (IGBT) fare better, but still require a clean power generator with 5% or less THD.

Here’s a video showing a Lincoln 180 MIG welder running on a 7250-watt generator:

Welding on Generator Power | Predator 9000

Generators for Welders

Compact and reliable, a modern portable generator will power your welder and other tools at any jobsite, and keep your lights and refrigerator on at home during power outages.

Conventional Generators for Welders

Essentially a motor-driven AC alternator, traditional general purpose generators make power in a single step. Spinning the alternator at 3600 RPM creates 120 volts with a frequency of 60 Hz.

Any variation to this speed causes voltage and frequency to fluctuate, resulting in harmonic distortion. Sure, the governor will try to maintain a steady RPM, but any significant change in load will cause a brief spike up or down.

While traditional transformer-based welders work well with conventional generators, the power does not meet clean standards. This style of generator should not power inverter-based welders needing 5% or less THD.

Preview
Factory Tested
Westinghouse WGen6000 Portable Generator 6000 Rated & 7500 Peak Watts, Gas Powered,...
Remote Start
Westinghouse Outdoor Power Equipment WGen9500 Heavy Duty Portable Generator 9500 Rated...
Dual Fuel
DuroMax XP12000EH Generator-12000 Watt Gas or Propane Powered Home Back Up & RV Ready, 50...
Welder
Westinghouse WGen6000 Portable Generator
Westinghouse WGen9500 Portable Generator
DuroMax XP12000EH Dual Fuel Generator
Highlights
For 120V and small 240V transformer welders. Each unit factory tested, Push-button electric start, VFT Data Center displays volt output, frequency, and lifetime hours
For all 120V and many 240V transformer welders up to 250A. Factory Tested, Remote Start Key Fob, Long-Lasting Cast Iron Sleeve With Automatic Low Oil Shutdown and Digital Hour Meter
For all 120V and many 240V transformer welders up to 250A. Runs on gas or propane. EPA and CARB approved emissions. All-metal construction.
Run/Start Watts
6000 / 7500W
9500 / 12500W
9500 / 12000W
Output Volts
120/240V
120/240V
120/240V
Outlets
(4) 120V GFCI 5-20R, (1) Transfer Switch Ready L14-30R 120/240V, (2) USB
(4) 120V GFCI 5-20R, (1) Transfer Switch Ready L14-30R 120/240V twist-lock, (1) 120/240V 14-50R, (2) USB ports
(2) 120V 20A GFCI, (1) 120V 30A, (1) 240V 30A, (1) 240V 50A
Start
Electric, Recoil
Electric, Remote, Recoil
Electric, Recoil
Weight
174 lbs.
220 lbs.
224 lbs.
Fuel Capacity
6.6 gal.
6.6 gal.
8.3 gal.
Run Time @ 50% Load (Est.)
13 hrs.
12 hrs.
8.8 hrs
Engine Size
420cc
457cc
457cc
Noise Level
72 dBA
74 dBA
74 dBA
THD
Less than 23%
Less than 23%
Less than 12%
Factory Tested
Preview
Westinghouse WGen6000 Portable Generator 6000 Rated & 7500 Peak Watts, Gas Powered,...
Welder
Westinghouse WGen6000 Portable Generator
Highlights
For 120V and small 240V transformer welders. Each unit factory tested, Push-button electric start, VFT Data Center displays volt output, frequency, and lifetime hours
Run/Start Watts
6000 / 7500W
Output Volts
120/240V
Outlets
(4) 120V GFCI 5-20R, (1) Transfer Switch Ready L14-30R 120/240V, (2) USB
Start
Electric, Recoil
Weight
174 lbs.
Fuel Capacity
6.6 gal.
Run Time @ 50% Load (Est.)
13 hrs.
Engine Size
420cc
Noise Level
72 dBA
THD
Less than 23%
Owner's Manual
More Information
Remote Start
Preview
Westinghouse Outdoor Power Equipment WGen9500 Heavy Duty Portable Generator 9500 Rated...
Welder
Westinghouse WGen9500 Portable Generator
Highlights
For all 120V and many 240V transformer welders up to 250A. Factory Tested, Remote Start Key Fob, Long-Lasting Cast Iron Sleeve With Automatic Low Oil Shutdown and Digital Hour Meter
Run/Start Watts
9500 / 12500W
Output Volts
120/240V
Outlets
(4) 120V GFCI 5-20R, (1) Transfer Switch Ready L14-30R 120/240V twist-lock, (1) 120/240V 14-50R, (2) USB ports
Start
Electric, Remote, Recoil
Weight
220 lbs.
Fuel Capacity
6.6 gal.
Run Time @ 50% Load (Est.)
12 hrs.
Engine Size
457cc
Noise Level
74 dBA
THD
Less than 23%
Owner's Manual
More Information
Dual Fuel
Preview
DuroMax XP12000EH Generator-12000 Watt Gas or Propane Powered Home Back Up & RV Ready, 50...
Welder
DuroMax XP12000EH Dual Fuel Generator
Highlights
For all 120V and many 240V transformer welders up to 250A. Runs on gas or propane. EPA and CARB approved emissions. All-metal construction.
Run/Start Watts
9500 / 12000W
Output Volts
120/240V
Outlets
(2) 120V 20A GFCI, (1) 120V 30A, (1) 240V 30A, (1) 240V 50A
Start
Electric, Recoil
Weight
224 lbs.
Fuel Capacity
8.3 gal.
Run Time @ 50% Load (Est.)
8.8 hrs
Engine Size
457cc
Noise Level
74 dBA
THD
Less than 12%
Owner's Manual
More Information
Advertisement

Inverter Generators for Welders

It turns out that the same inverter technology that gives welders a stable output also helps generators make clean power in three steps:

  • Generate high frequency AC with an engine-driven alternator
  • Convert AC to DC current
  • Invert DC current into a lower and very stable AC supply current

Starting with high energy, AC current allows the inverter more control over the final output. The result is a stable sine wave with low THD (under 5%) that is perfect for both inverter and conventional welders, along with other sensitive electronics.

Preview
Ultra-Quiet
Generac 7127 iQ3500-3500 Watt Portable Inverter Generator Quieter Than Honda, Orange/Black
Full Outlet Bank
Powerhorse 4000 Watt Portable Generator
Digital Smart Gauge
Champion Power Equipment 100519 6250-Watt Open Frame Inverter with Quiet Technology
Welder
Generac 7127 iQ3500-3500 Watt Portable Inverter Generator
Powerhorse Portable 4000 Watt Generator
Champion Power 6250W Inverter Generator
Highlights
For many small 120V welders up to 100A. Electric start with included battery, parallel ready, smart gauge displays wattage and run time remaining.
For many small 120V welders up to 100A. Clean, reliable electricity. 100% copper winding allows continuous operation without overheating. Automatic voltage regulator with voltmeter and low oil shutdown.
For small 120V welders up to 165A. Quiet running clean power. 3-year warranty. Monitor voltage, frequency and operating hours from Intelligauge
Run/Start Watts
3000 / 3500W
3100 / 4000W
5000 / 6250W
Output Volts
120V
120/240V
120/240V
Outlets
(1) 120V 50A 14-50R, (1) 120V 30A TT-30R (RV), (2) USB
(8) 120V, 20A, (1) 120/240V 30A, (1) 12V DC
(1) 120/240V 30A (L14-30R), (4) GFCI 120V 20A (5-20R), (1) 12V DC
Start
Electric / Recoil Pull Start
Recoil
Recoil
Weight
109 lbs.
115 lbs.
121 lbs.
Fuel Capacity
2.6 gal.
3.9 gal.
4.2 gal.
Run Time @ 50% Load (Est.)
8.9 hrs.
10.6 hrs.
8.4 hrs
Engine Size
212cc
212cc
301cc
Noise Level
N/A
n/a
69 dBA
THD
Less than 3%
Less than 5%
Less than 3%
Ultra-Quiet
Preview
Generac 7127 iQ3500-3500 Watt Portable Inverter Generator Quieter Than Honda, Orange/Black
Welder
Generac 7127 iQ3500-3500 Watt Portable Inverter Generator
Highlights
For many small 120V welders up to 100A. Electric start with included battery, parallel ready, smart gauge displays wattage and run time remaining.
Run/Start Watts
3000 / 3500W
Output Volts
120V
Outlets
(1) 120V 50A 14-50R, (1) 120V 30A TT-30R (RV), (2) USB
Start
Electric / Recoil Pull Start
Weight
109 lbs.
Fuel Capacity
2.6 gal.
Run Time @ 50% Load (Est.)
8.9 hrs.
Engine Size
212cc
Noise Level
N/A
THD
Less than 3%
Owner's Manual
More Information
Full Outlet Bank
Preview
Powerhorse 4000 Watt Portable Generator
Welder
Powerhorse Portable 4000 Watt Generator
Highlights
For many small 120V welders up to 100A. Clean, reliable electricity. 100% copper winding allows continuous operation without overheating. Automatic voltage regulator with voltmeter and low oil shutdown.
Run/Start Watts
3100 / 4000W
Output Volts
120/240V
Outlets
(8) 120V, 20A, (1) 120/240V 30A, (1) 12V DC
Start
Recoil
Weight
115 lbs.
Fuel Capacity
3.9 gal.
Run Time @ 50% Load (Est.)
10.6 hrs.
Engine Size
212cc
Noise Level
n/a
THD
Less than 5%
Owner's Manual
More Information
Digital Smart Gauge
Preview
Champion Power Equipment 100519 6250-Watt Open Frame Inverter with Quiet Technology
Welder
Champion Power 6250W Inverter Generator
Highlights
For small 120V welders up to 165A. Quiet running clean power. 3-year warranty. Monitor voltage, frequency and operating hours from Intelligauge
Run/Start Watts
5000 / 6250W
Output Volts
120/240V
Outlets
(1) 120/240V 30A (L14-30R), (4) GFCI 120V 20A (5-20R), (1) 12V DC
Start
Recoil
Weight
121 lbs.
Fuel Capacity
4.2 gal.
Run Time @ 50% Load (Est.)
8.4 hrs
Engine Size
301cc
Noise Level
69 dBA
THD
Less than 3%
Owner's Manual
More Information
Preview
Compact Power
Champion Power Equipment 100520 8750-Watt DH Series Open Frame Inverter, Electric Start
40A Outlet
Powerhorse 13,000 Watt Portable Generator — Electric Start
Welder
Champion 8750 Watt Inverter Generator
Powerhorse 13000W Portable Generator
Highlights
For 120V and small 240V welders, including inverter. Clean power digital hybrid inverter generator. EPA certified and CARB compliant. Small footprint for output.
For 120V and many 240V welders up to 250A, including inverter. 8 clean-power outlets. 100% copper-wound generator head allows continuous operation without overheating.
Run/Start Watts
7000 / 8750W
10000 / 13000W
Output Volts
120/240V
120/240V
Outlets
(1) 120/240V 30A Locking (L14-30R), (4) GFCI 120V 20A (5-20R), (1) 12V DC Automotive
(4) 120V 20A, (1) 120V 30A locking, (1) 120/240V 30A locking, (1) 120/240V 40A, (1) 12V DC
Start
Electric
Electric
Weight
155 lbs.
324 lbs.
Fuel Capacity
4.2 gal.
12.6 gal.
Run Time @ 50% Load (Est.)
6 hrs.
15 hrs.
Engine Size
420cc
622cc
Noise Level
72 dBA
76.8 dBA
THD
Less than 3%
Less than 5%
Owner's Manual
More Information
Compact Power
Preview
Champion Power Equipment 100520 8750-Watt DH Series Open Frame Inverter, Electric Start
Welder
Champion 8750 Watt Inverter Generator
Highlights
For 120V and small 240V welders, including inverter. Clean power digital hybrid inverter generator. EPA certified and CARB compliant. Small footprint for output.
Run/Start Watts
7000 / 8750W
Output Volts
120/240V
Outlets
(1) 120/240V 30A Locking (L14-30R), (4) GFCI 120V 20A (5-20R), (1) 12V DC Automotive
Start
Electric
Weight
155 lbs.
Fuel Capacity
4.2 gal.
Run Time @ 50% Load (Est.)
6 hrs.
Engine Size
420cc
Noise Level
72 dBA
THD
Less than 3%
Owner's Manual
More Information
40A Outlet
Preview
Powerhorse 13,000 Watt Portable Generator — Electric Start
Welder
Powerhorse 13000W Portable Generator
Highlights
For 120V and many 240V welders up to 250A, including inverter. 8 clean-power outlets. 100% copper-wound generator head allows continuous operation without overheating.
Run/Start Watts
10000 / 13000W
Output Volts
120/240V
Outlets
(4) 120V 20A, (1) 120V 30A locking, (1) 120/240V 30A locking, (1) 120/240V 40A, (1) 12V DC
Start
Electric
Weight
324 lbs.
Fuel Capacity
12.6 gal.
Run Time @ 50% Load (Est.)
15 hrs.
Engine Size
622cc
Noise Level
76.8 dBA
THD
Less than 5%
Owner's Manual
More Information

Tips for Welding with a Generator

  • Size the generator to your welder:
    • Know your welders wattage requirement for full operation. From the data plate, multiply I1max (max amps) x U₁ (volts) = maximum watts.
    • Include watts for accessory items that you might need while you weld: lights, fans, etc.
    • Allow for 3.5% of power loss per every 1000 feet of altitude.
    • Choose a generator capable of producing at least 25-30% more watts than you need for best results.
    • Undersized generators can have larger voltage and frequency spikes. This is hard on the generator and welder. It also makes welding more difficult.
    • A generator running at 50-60% capacity will handle load fluctuations better than one running at 90%+.
  • Starting and Shutdown: Unplug your welder before starting or shutting down the generator.
  • Auto-Idle: Disable off any fuel saving feature so the generator always operates at full speed while connected to the welder.
  • Fuel: Keep topped up with fresh fuel. You never want the generator to stop running while connected to the welder—especially while you’re running a bead.
  • Maintenance: Keep the generator operating properly. Don’t operate your welder on a generator that isn’t running well.
  • Power/Extension Cords: Don’t use cords that are undersized or in poor condition. Here’s how to size a welder extension cord.
Forney Easy Weld 140 FC-i Owner’s Manual

Will a 3000 watt generator run a welder?

A 3000-watt generator at 120-volt output can generate 25 amps of current (Amps = Watts/Volts). While many 120V welders draw more amps at full power startup, a small welder with 90 to 100-amp output should work.

Will a 5000 watt generator run a welder?

Yes, a 5000-watt generator is ideal for 120V welders that draw less than 40 amps at startup, which includes most welders up to 160 amps. But most 240V machines need more than 5000 watts to run at full capacity.

what size generator for 140 amp welder

What size generator to run a 140 amp welder?

Many welders rated for 140-amp output will draw less than 30 amps at startup, but some need nearly 40 amps.

To be safe, get the I1max (max amps) rating and multiply it by the voltage for the maximum watts. Expect to need 3600 to 4800 watts for full operation of a 140-amp welder.

Conclusion

Some of you are researching what size generator to use for an existing welder, and others have a generator and want to know what size of welder it can handle.  

Either way, you can now make an informed decision based on the minimum wattage required to operate a welder at full capacity.

While it’s ideal to power your welder with a generator having 30-50% more capacity than you need, you may decide that a welder/generator combo allowing only 80% of welder capacity is right for you.

You’ll make the call based on your situation. Sizing a generator for welding forces us to balance power, price, and portability.

Dave Jones
 

Dave began welding to repair equipment used in his small business. Now as a hobby, he enjoys researching, testing and writing on welding topics. Other interests include photography, RVing and just about anything to do with dogs—especially retrievers. Reach him at dave@welditu.com.