Size a generator for a welder from the welder’s AC input requirements at the welding output you plan to use, then check generator compatibility and transient response. Welding output amperes are not the amperes the generator must supply.
Open the Welder Generator Size Calculator.
Find the welder’s input specifications
Start with the exact model’s manual and rating plate. Record its permitted input voltage, phase, frequency, and input demand at the relevant welding output. Also look for specific generator requirements or limitations; these are more useful than a generic wattage multiplier.
A welder may support multiple supply voltages but deliver different maximum welding output on each. A machine advertised with one large welding-amp figure may require its higher-voltage input to reach that setting. Match the planned job to the specifications for the actual supply.
Distinguish maximum input current from effective current values used for particular installation calculations. A duty-cycle-adjusted value does not necessarily describe the instantaneous input while the arc is on. Use the manufacturer’s generator guidance and the correct table definition when selecting the source.
If the manual provides a generator minimum, verify the conditions attached to it: output setting, continuous versus maximum generator rating, voltage quality, and operating mode. Do not assume the same minimum applies to every process or output available on the welder.
Keep welding output and supply input separate
A welding circuit may carry a high current at a relatively low arc voltage. The generator supplies AC input at a much higher voltage, and the welder converts that power into the required welding output.
For an illustrative welding setting of 150 A at an average 25 V arc voltage, output power is 150 × 25 = 3,750 W. That does not mean the AC input is 150 A, and it does not mean a 3,750 W generator is automatically sufficient.
The welder consumes more real input power than its useful welding output because conversion is not perfectly efficient. Its input apparent power can be higher again when power factor is below 1. The generator must be able to supply the relevant real power, current, and changing demand.
A welding-amp label alone therefore lacks several necessary inputs. Two welders delivering similar welding current can have different arc voltage, efficiency, power factor, and input behavior. A universal generator size based only on welding amps would hide those differences.
Calculate from AC voltage and input current
For single-phase input, apparent demand in kVA equals supply volts multiplied by input amperes, divided by 1,000. For balanced three-phase input, use square root of three multiplied by line-to-line volts and line current, divided by 1,000.
Suppose a hypothetical single-phase welder specifies 24 A input at 240 V for the intended setting. Its apparent input is 240 × 24 ÷ 1,000 = 5.76 kVA. If its applicable input power factor is 0.90, real input is 5.76 × 0.90 = 5.184 kW.
If a planning margin of 25% is selected, those figures become 7.20 kVA and 6.48 kW respectively. Keep both checks visible. A generator rated only in kW does not automatically have any desired kVA capability at every power factor.
This margin is an example planning allowance, not a manufacturer rule. The actual generator selection may need more or less depending on the approved equipment pairing and response. The welder’s documented input and generator requirements remain the primary evidence.
Use the output-based calculator as a cross-check
The embedded calculator starts from welding output current and arc voltage. It divides their product by efficiency to estimate real input, divides by power factor to estimate apparent input, and then applies the selected reserve factor.
For 150 A, 25 V, 85% efficiency, and 0.80 power factor:
Welding output = 150 × 25 = 3.75 kW.
Estimated AC real input = 3.75 ÷ 0.85 = 4.412 kW.
Estimated AC apparent input = 4.412 ÷ 0.80 = 5.515 kVA.
With a 25% reserve factor, the apparent-power estimate is approximately 6.89 kVA. The corresponding real-power planning amount is approximately 5.51 kW. At 240 V single phase, 6.89 kVA corresponds to about 28.7 A.
| Calculation stage | Illustrative result |
|---|---|
| Welding output | 3.75 kW |
| Estimated real AC input | 4.41 kW |
| Estimated apparent AC input | 5.51 kVA |
| Apparent-power estimate with 25% reserve | 6.89 kVA |
| Corresponding real-power estimate with reserve | 5.51 kW |
Do not call the 6.89 kVA result 6.89 kW without considering power factor. Also avoid applying efficiency and power factor again to a manufacturer input-kVA specification; those effects are already represented in that input figure.
Understand the limits of estimated arc voltage
The calculator is only as useful as the entered values. An assumed 25 V arc is an illustration, not a fixed operating voltage for every welding process, electrode, wire, or current setting.
If a machine publishes input data at the desired output, use those data directly rather than reconstructing them from a guessed arc voltage. Published efficiency and power-factor values may also apply only at specified operating points, not across the entire adjustment range.
Power-factor correction can affect input demand, but the presence of an inverter does not guarantee unity power factor. Likewise, a transformer-based design should not be assigned a universal power factor without supporting data.
If the output-based estimate disagrees with the manual, check whether the inputs describe the same operating condition. Process choice, input voltage, output current, arc voltage, and duty cycle may differ. Do not average unlike values to make them agree.
Check voltage behavior and generator operating mode
Welding can impose changing input demand as the arc starts and the work progresses. The generator must maintain an acceptable supply within the welder’s requirements during those changes, not merely support an average watt figure.
Miller provides a manual lookup by model or serial number. Obtain the matching instructions and check their generator requirements rather than transferring a numerical supply limit from another welder.
Some equipment instructions restrict generator automatic-idle or economy operation. Follow the particular welder and generator instructions instead of assuming that the quietest setting is suitable for a rapidly changing load. An adequate continuous rating does not by itself establish acceptable voltage recovery.
Also confirm that the chosen generator provides the required voltage and phase through an appropriately rated connection. A large 120 V-only generator cannot serve a 240 V-only welder just because its total watts exceed the estimate.
Add other tools without double-counting the welder
A jobsite generator may also run lighting, ventilation equipment, grinders, or a compressor. Add the loads expected to operate while welding, and assess motor starts separately where they can overlap.
Suppose the welder needs an illustrative 5.2 kW real input while a 0.3 kW lighting and ventilation load continues. Their combined real demand is 5.5 kW. If a 1.2 kW grinder is only used between welds, it does not necessarily add to that particular simultaneous welding case.
A compressor controlled by its pressure switch is different because it can start automatically. If it may start during welding, include that scenario or use a reliable operating arrangement that prevents overlap. Simply intending to avoid simultaneous use is weaker than a plan that actually controls it.
Preserve current and kVA limits as well as kW totals. Other tools can change power factor and phase allocation, so their combined effect cannot always be described by a single sum of nominal watts.
Do not reduce source capacity by the welding duty cycle
Duty cycle describes an allowed pattern of welding and cooling at stated conditions. It does not mean the generator can supply only a fraction of the welder’s on-arc demand.
For example, a welder drawing 6 kVA during the arc still requires that input while welding, even if its operating schedule includes substantial cooling time. Multiplying 6 kVA by a 40% duty cycle to select a 2.4 kVA generator confuses average use with instantaneous supply capability.
Duty cycle is relevant to work planning, average energy, and possibly fuel use. Source sizing still needs the demand during the active portion and the generator’s ability to support it. Respect the welder’s thermal limits independently of the generator rating.
Keep the connection equipment, cords, and protective devices appropriate to the exact installation. A calculator result is not an extension-cord size or a reason to bypass a breaker. If the supply trips or voltage falls out of range, diagnose the mismatch rather than increasing protection to force operation.
Record the conditions behind the chosen size
Before selecting a generator, write down the welder model, intended process and output setting, input supply, documented current or kVA, manufacturer generator guidance, and other simultaneous loads. Include the selected reserve and any site or fuel adjustment.
That record makes future changes easier to assess. A larger electrode, higher output setting, additional compressor, or different input voltage may change the requirement even when the welder cabinet remains the same.
Use the generator’s continuous rating as the sustained basis and verify any short-duration rating on its own terms. The strongest result is a supported equipment combination for a defined job, with both electrical demand and operating behavior checked.