Industrial Generator Sizing Guide: How to Calculate kW, kVA & Amps
Sizing an industrial generator is not as simple as adding up the wattage of your lightbulbs. Heavy machinery requires massive amounts of inrush current to start, and commercial facilities use 3-phase power with complex Power Factors (PF). Undersizing a generator will lead to voltage dips and equipment failure, while oversizing leads to "wet stacking" and wasted capital. This guide breaks down the exact math needed to size a commercial generator correctly.
Understanding kW vs. kVA
In residential applications, generators are usually sized in Watts or Kilowatts (kW). In industrial applications, generators are sized in kilo-Volt-Amperes (kVA). The difference between the two is the Power Factor (PF).
- kW (Real Power): The actual power that is converted into useful work (heat, motion, light).
- kVA (Apparent Power): The total power required by the system, including the power required to sustain magnetic fields in induction motors (Reactive Power).
kW = kVA × Power Factor (PF)
kVA = kW ÷ Power Factor (PF)
For standard industrial 3-phase generators, the nominal Power Factor is almost always exactly 0.8. This means a 100 kVA generator will produce exactly 80 kW of Real Power.
How to Calculate Amps to kVA
If you are reading the data plates on your machinery, you will likely see a voltage and an amperage rating. To size the generator, you must convert Amps to kVA.
Single Phase Formula
kVA = (Volts × Amps) ÷ 1,000
Three-Phase Formula
Because 3-phase power delivers three alternating currents offset by 120 degrees, you must multiply by the square root of 3 (approx. 1.732).
kVA = (Volts × Amps × 1.732) ÷ 1,000
Factoring in Motor Starting Surge
The biggest mistake engineers make when sizing a generator is calculating the Running Watts and ignoring the Starting Watts (Inrush Current). When an electric induction motor starts (like an air compressor, chiller, or conveyor), it draws 3 to 6 times its normal running current for a few seconds.
If your generator is not sized to handle this surge (SkVA), the voltage will drop catastrophically, the motor will stall, and the generator breaker will trip.
To prevent this, use a staggered startup sequence (so all motors don't start at the exact same millisecond), and ensure your generator's SkVA rating exceeds the highest single motor starting surge plus the running load of all other equipment.
Applying the 20% Safety Margin
Generators should not run at 100% capacity continuously. Doing so stresses the alternator and drastically increases exhaust temperatures. Conversely, diesel generators should not run below 30% load, as this causes unburned fuel to build up in the exhaust (Wet Stacking).
The industry standard is to calculate your total maximum simultaneous load, and then add a 20% to 25% safety margin. This ensures the generator runs comfortably at 75%-80% capacity during peak facility demand.
Rather than doing this math by hand, you can use our Industrial Generator Sizing Calculator to handle the 3-phase math and inrush multipliers automatically.
Frequently Asked Questions
What is a Power Factor of 0.8?
In AC circuits, especially 3-phase industrial power, induction motors cause voltage and current to fall out of phase. A Power Factor of 0.8 means only 80% of the Apparent Power (kVA) is doing real work (kW), while 20% is used to maintain magnetic fields in the motors.
Can I run a diesel generator at 10% load?
No. Running a commercial diesel generator at a very low load (typically under 30%) prevents the engine from reaching proper operating temperature. This causes "wet stacking," where unburned fuel and carbon build up in the exhaust, leading to engine damage.