Size a generator for the welder’s electrical demand while the arc is active, then use duty cycle to plan how long that load will remain on the generator. A low duty cycle reduces average energy use, but it does not make a demanding welding burst easier to supply.
Open the Welding Generator Size Calculator.
What does a welder’s duty cycle actually mean?
Duty cycle describes an operating limit at a stated welding output and under the manufacturer’s specified conditions. A machine rated at 40% duty cycle over ten minutes permits four minutes of welding followed by six minutes of cooling at that rating. Miller explains the ten-minute basis and the importance of reading the rating at the required amperage in its duty-cycle guide.
The percentage belongs to a particular point on the welder’s performance curve. A rating printed beside a lower current setting may not apply when you turn the current up. Before calculating generator size, record the welding process, desired output current, and applicable duty-cycle rating together.
Also distinguish the welder’s thermal limit from your expected work pattern. You might spend most of an hour positioning steel, making short tacks, and cleaning joints. That creates a light average workload without changing the power needed during each tack. Conversely, a long seam can hold the electrical demand near its active value for several minutes even if the hour’s total welding time is modest.
The generator experiences both conditions: brief changes when the arc starts and a sustained load while welding continues. Neither condition can be represented adequately by multiplying active watts by the duty-cycle percentage.
Start with input specifications rather than output amperage
The current at the welding terminals differs from the AC current taken from the generator. Multiplying a 200 A welding setting by a 240 V supply is incorrect because those numbers describe different parts of the system.
If manufacturer input data are available for the intended setting, use them first. Look for input voltage, phase, rated or maximum input current, input kW or kVA, power factor, and any explicit generator recommendation. Some nameplates also distinguish maximum input current from an effective current used for conductor thermal calculations. Those values serve different purposes; the smaller effective figure is not automatically the generator’s instantaneous load.
If input power is unavailable, a first estimate is:
Arc output watts = welding current × arc voltage
AC real input watts ≈ arc output watts ÷ welder efficiency
Express efficiency as a decimal. An 85% assumption becomes 0.85. This calculation does not identify power factor, waveform quality, auxiliary consumption, or the highest demand during arc initiation. It is a starting estimate that needs comparison with the welder’s documentation.
Arc voltage also matters. Two jobs at the same welding amperage can have different output power when their arc voltages differ. Do not substitute open-circuit voltage for operating arc voltage.
An illustrative 180 A welding example
Suppose a job uses 180 A at an illustrative 25 V arc voltage. Assume 85% conversion efficiency solely for this example. Arc output is 180 × 25 = 4,500 W, and estimated active AC input is 4,500 ÷ 0.85 = approximately 5,294 W.
Now suppose the welder permits a 40% duty cycle at that output. Ignoring standby consumption, average input over the full ten-minute cycle would be about 5,294 × 0.40 = 2,118 W. However, the generator must still provide approximately 5,294 W throughout each four-minute welding period.
| Quantity | Illustrative calculation | Result |
|---|---|---|
| Arc output | 180 A × 25 V | 4,500 W |
| Active real input | 4,500 ÷ 0.85 | 5,294 W |
| Ten-minute average, ignoring standby | 5,294 × 0.40 | 2,118 W |
| Active input plus 500 W accessories | 5,294 + 500 | 5,794 W |
| Planning value with 25% allowance | 5,794 × 1.25 | 7,243 W |
The final row is a planning value, not proof that a generator bearing a particular marketing wattage will operate the welder. Its usable continuous output, receptacle capacity, voltage, power quality, and transient behavior still need checking.
If the welder’s actual active input power factor were 0.80, its apparent input would be about 5,294 ÷ 0.80 = 6,618 VA before accessories. The generator must satisfy its current and apparent-power limits as well as its real-power limit. Do not treat the assumed efficiency and power factor as interchangeable discounts.
How the calculator handles duty cycle
The embedded calculator estimates active input from welding current, arc voltage, and efficiency. It then multiplies that value by a duty-related factor and a selected headroom factor.
Its duty factor is 0.85 + 0.15 × duty-cycle fraction. At 40% duty cycle, that factor is 0.91. With the example’s inputs and 25% headroom, the result is approximately 5,294 × 0.91 × 1.25 = 6,022 W. The tool does not include the example’s separate 500 W accessory load.
This factor is a heuristic. It is not the same as a thermal duty-cycle rating or the actual average energy calculation. More importantly, reducing the active input estimate because the welder rests between welds can obscure the demand while the arc is established.
For a more conservative comparison within the tool, run the same output settings at 100% duty cycle. That removes its duty-related reduction and gives approximately 6,618 W with the same 25% headroom. This does not authorize continuous welding beyond the machine’s own rating; it simply avoids lowering the power estimate because of cooling pauses.
Keep the maximum-input and manufacturer-generator checks alongside either result. A calculator selection cannot establish compatibility with a particular inverter welder.
Short bursts and sustained welding create different checks
For repair work, the most troublesome event may be repeated arc initiation while another tool is running. The operator might weld only occasionally, but each attempt still produces an abrupt load change. A grinder, extraction fan, light, or compressor needs its own entry on the load schedule.
For sustained fabrication, focus on the generator’s rated output over the welding period and its applicable operating rating. A headline surge rating is not a four-minute continuous rating. Cooling pauses in the welder also do not alter the generator manufacturer’s permitted operating conditions.
Create at least two realistic scenarios. In the first, the welder operates while essential accessories run. In the second, a motor-driven accessory starts while the welder is already active. If the operating plan ensures those events cannot overlap, write that restriction into the work plan rather than silently assuming it.
Generator response to sudden loads depends on the complete machine and its controls, not just its rated kW. Cummins discusses this system behavior in its generator application manual. For a welder, obtain compatibility guidance from both equipment manufacturers where available.
Check the connection and available output
A generator can have enough total watts and still lack a suitable outlet. Confirm the welder’s input voltage and phase, the receptacle’s current rating, the compatible connection arrangement, and the generator’s output rating on the fuel you intend to use.
For an illustrative 240 V single-phase input of 6.6 kVA, current is approximately 6,600 ÷ 240 = 27.5 A. That is an apparent-power calculation, so there is no additional efficiency multiplier. The real welder may require a different current and protection arrangement according to its manual.
Cable voltage drop can also affect the result. Record the actual cable length and the approved cable arrangement instead of assuming that power available at the generator reaches the welder unchanged. Have the electrical connection and protection checked for the equipment and location involved.
Outdoor engine placement is part of that cable planning. CPSC directs users to operate portable generators outdoors, at least 20 feet from homes, with exhaust directed away; an open garage door does not make indoor operation safe. See its carbon-monoxide guidance.
Questions to resolve before choosing a generator
Can a smaller generator work if I only make tacks?
Possibly, if the chosen welding setting stays within the generator’s verified capability. Tack duration alone does not establish that capability. Lower current and appropriate arc voltage may reduce active demand, but the result must remain suitable for the weld and within both manuals.
Does 100% duty cycle mean the welder always uses maximum power?
No. It describes permitted operating time at a specified output. Actual input depends on the welding setting and operating state. A welder with a 100% rating at one setting can still have a lower permitted duty cycle at a higher output.
What should I send a generator supplier?
Send the exact welder model, input specifications, intended process and settings, duty-cycle information, accessory schedule, supply voltage and phase, and expected cable arrangement. Ask the supplier to evaluate both active operation and abrupt load changes. A photograph of only the front-panel amperage control leaves out much of the information needed.
Recalculate when welding settings change
A lower-current repair setting can produce a different result from the maximum setting used to compare machines. For an illustrative 120 A at 22 V, arc output is 2,640 W. At the same assumed 85% efficiency, active real input is approximately 3,106 W before accessories, substantially below the earlier 180 A example.
That comparison is valid only for the defined setting and process. Do not label a generator suitable for every job on the welder because it supports one lower-demand operation. Retain separate operating limits where the manufacturer permits the combination, and make the supported setting visible to whoever uses the equipment.
References
- Miller: duty cycle and operating ratings.
- Cummins T-030 generator application manual.
- CPSC carbon-monoxide and portable-generator guidance.