Estimate the generator capacity required for a welding machine from welding output, electrical efficiency, power factor, additional loads and generator sizing margins.
A welding machine consumes electrical input power to produce welding output. Generator sizing should therefore be based on the electrical demand of the welding machine rather than welding output alone.
When the welding machine's input power is not available, the calculator estimates input power using the entered electrical efficiency.
The calculator then converts real electrical load into apparent generator load using generator power factor.
Finally, transient and general sizing margins are applied.
Generator capacity is commonly expressed in kVA because the alternator must supply voltage and current regardless of the power factor of the connected equipment.
A welding machine with a lower power factor can therefore require more generator kVA for the same real power consumption.
Actual welding duty cycle indicates how much of the operating period the welding arc is active. It is useful for estimating average energy consumption but should not simply be used to reduce the generator's peak capacity.
The generator should still be capable of supplying the required welding load when the machine is operating. Duty cycle can help estimate average generator utilization during a shift.
If the generator also powers other equipment at the same time as the welder, those loads must be included.
| Equipment | Include in Generator Calculation? |
|---|---|
| Welder | Yes — primary electrical load. |
| Angle grinder | Yes, when it can operate simultaneously. |
| Air compressor | Yes, including its motor starting demand. |
| Lighting | Yes. |
| Ventilation equipment | Yes. |
| Battery chargers | Yes. |
Motors and some electrical equipment can draw substantially more current during startup than during normal operation.
Compressors, grinders, fans and pumps can therefore require additional generator capacity when they start while the welder is operating.
The transient margin in this calculator is a planning allowance. Actual starting requirements should be taken from the equipment manufacturer's specifications whenever available.
Selecting a generator exactly equal to the calculated load leaves little capacity for load variation, transient demand or additional equipment.
The calculator rounds the final result upward to a practical 5 kVA increment.
For single-phase electrical systems, apparent power can be estimated from voltage and current using:
For balanced three-phase systems:
For generator selection, the welding machine's actual input specification remains preferable to estimating input from welding output.
How big of a generator do I need for a welder? It depends on the welding machine's electrical input requirement. Input kVA or maximum input current from the machine specification is preferable to estimating from welding amps alone.
Can generator size be calculated from welding amps? Yes, as an estimate when welding voltage, efficiency and power factor are known. The manufacturer's input specification is more accurate.
Why are generators rated in kVA? Generators are rated in apparent power because their alternators supply voltage and current, while power factor determines the relationship between real and apparent power.
Does welding duty cycle reduce generator size? Not necessarily. Duty cycle can reduce average energy consumption, but the generator still needs sufficient capacity for the welding machine's operating demand.
Should other tools be included? Yes. Any equipment that may operate simultaneously should be included.
Does an air compressor affect generator sizing? Yes. Its motor can require considerably more current during startup than during normal operation.
Is the calculated generator size exact? No. It is an engineering estimate. Final selection should use actual machine specifications and manufacturer recommendations.
Does generator power factor matter? Yes. Lower power factor increases the kVA required for a given real power load.
Should welder output power or input power be used? Input power is preferred. Output power can be used for an estimate when efficiency is known.
Can a generator operate an inverter welding machine? Many generator and inverter-welder combinations can work together, but compatibility depends on voltage regulation, waveform, capacity and manufacturer requirements.