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Evgenii Konkin
Evgenii Konkin

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How Power Factor Turns a 100 kVA Generator Into 80 kW of Usable Power

#electrical #power #engineering #tutorial

A generator nameplate shows two numbers: kVA and kW. They are not the same thing.

The difference between them is power factor.

The conversion formula is:

kW = kVA × PF
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Where:

kW = real power (useful work)
kVA = apparent power (total demand on the source)
PF = power factor of the connected load (0 to 1.0)
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Power factor tells you what fraction of the current flowing through the circuit is actually doing useful work. The rest is reactive current. It flows through the conductors, heats up the cables, occupies transformer capacity, and performs zero work. The source has to supply it, the cables have to carry it, but the load gives it back every half cycle.

For resistive loads, PF = 1.0. For induction motors, PF is typically 0.75 to 0.90. For mixed commercial buildings, 0.80 to 0.85 is a common range.

Example: generator sizing for a chiller plant

A mechanical contractor needs standby power for a chiller plant. The equipment list:

Equipment                kW      PF      kVA
Chiller compressor       150     0.85    176.5
Chilled water pumps       30     0.82     36.6
Condenser water pumps     25     0.82     30.5
Cooling tower fans        15     0.80     18.8
Controls and lighting     10     0.95     10.5
─────────────────────────────────────────────
Total                    230     0.843   272.9
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Each kVA value is calculated as:

kVA = kW / PF
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For the chiller compressor:

kVA = 150 / 0.85
kVA = 176.5
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The composite power factor of the entire plant is:

PF = 230 kW / 272.9 kVA
PF = 0.843
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Total apparent power demand is 272.9 kVA.

The sizing mistake

If you order a 250 kVA generator, you might think you have 20 kW of headroom.

Real power demand:     230 kW
Generator kW rating:   250 kVA × 0.80 PF = 200 kW (typical rating)
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But the apparent power demand tells a different story:

Apparent power demand: 272.9 kVA
Generator capacity:    250 kVA
Deficit:               22.9 kVA over limit
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The 250 kVA generator is oversized by 20 kW but undersized by 23 kVA. It will trip on overcurrent even though the kW load is within rating.

The overcurrent protection sees 272.9 kVA against a 250 kVA limit and does exactly what it is supposed to do.

Correct selection 

kVA required = 230 / 0.843 = 273 kVA minimum
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A 300 kVA generator is the correct selection. That gives you 273 kVA demand against 300 kVA capacity, plus margin for motor starting current which the steady-state calculation does not include.

The nameplate trap

Some manufacturers rate generators at 0.80 PF:

"250 kVA / 200 kW" = manufacturer already applied 0.80
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Other manufacturers, especially in the European market, rate at unity PF:

"250 kVA / 250 kW" = assumes all loads are resistive
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Same kVA rating. Very different real capacity for motor loads.

Always check whether the nameplate kW is at rated PF or at unity.

Motor starting makes it worse

Motor starting current can draw 5 to 7 times the running current for several seconds. During that inrush, the power factor drops dramatically, sometimes below 0.30.

Running:  150 kW at PF 0.85 = 176 kVA
Starting: 150 kW × 6 inrush at PF 0.30 = approx 3,000 kVA transient
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The generator must handle that transient kVA demand without voltage collapse.

This is why generator sizing requires more than just adding up nameplate kW. But the first step, before you even get to starting analysis, is making sure you are not confusing kVA with kW.

What the kVA to kW conversion does not cover

A correct kVA to kW conversion does not mean the generator is fully sized.

It does not verify:
  Motor starting kVA
  Harmonic distortion
  Transient response
  Step loading sequence
  Altitude or temperature derating
  Fuel system capacity
  Manufacturer acceptance
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The kVA to kW check is a first-pass screen.

It tells you whether the source capacity matches the load demand in both real and apparent power.

It does not prove the full system design is complete.

Practical takeaway

Before ordering a generator, transformer, or UPS for a mixed load:

1. List all equipment with kW and power factor
2. Calculate kVA for each: kVA = kW / PF
3. Sum the kVA column (not just the kW column)
4. Compare total kVA to the source kVA rating
5. Check whether the manufacturer's kW rating is at 0.80 PF or unity
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That is enough to catch the most common sizing mistake before it becomes a commissioning problem.

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