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Typical values: 0.8 for motors, 0.95-1.0 for resistive loads
P(W) = S(kVA) × 1000 × PF
Where:
kVA (kilovolt-amperes) represents apparent power, while watts represent real power. The power factor determines how much of the apparent power is actually doing useful work. A power factor of 1.0 means all power is real power; lower values indicate reactive power.
| kVA | PF=0.8 | PF=0.85 | PF=0.9 | PF=0.95 | PF=1.0 |
|---|---|---|---|---|---|
| 1 kVA | 800 W | 850 W | 900 W | 950 W | 1,000 W |
| 2 kVA | 1,600 W | 1,700 W | 1,800 W | 1,900 W | 2,000 W |
| 5 kVA | 4,000 W | 4,250 W | 4,500 W | 4,750 W | 5,000 W |
| 10 kVA | 8,000 W | 8,500 W | 9,000 W | 9,500 W | 10,000 W |
| 15 kVA | 12,000 W | 12,750 W | 13,500 W | 14,250 W | 15,000 W |
| 20 kVA | 16,000 W | 17,000 W | 18,000 W | 19,000 W | 20,000 W |
| 25 kVA | 20,000 W | 21,250 W | 22,500 W | 23,750 W | 25,000 W |
| 50 kVA | 40,000 W | 42,500 W | 45,000 W | 47,500 W | 50,000 W |
| 100 kVA | 80,000 W | 85,000 W | 90,000 W | 95,000 W | 100,000 W |
kVA to watts conversion determines the real (usable) power output from a given apparent power rating. In AC electrical systems, apparent power (measured in kilovolt-amperes, kVA) represents the total power flowing in the circuit, while real power (measured in watts, W) is the portion that performs actual work. The relationship is governed by the power factor (PF): Watts = kVA × 1,000 × PF. The power factor ranges from 0 to 1 and reflects how efficiently the electrical load converts apparent power into useful work. Purely resistive loads like heaters have a power factor of 1.0, meaning all apparent power becomes real power. Inductive loads such as motors and transformers typically have power factors between 0.75 and 0.90, meaning a significant portion of the apparent power is reactive and does no useful work. This conversion is essential for sizing generators, transformers, UPS systems, and understanding utility bills per IEEE and NEC standards.
Locate the apparent power rating on the equipment nameplate. Generators, transformers, and UPS systems display their capacity in kVA. For example, a standby generator might be rated at 20 kVA.
Check the load power factor from equipment specs or measure it with a power analyzer. Use 0.8 for motor-heavy loads, 0.85–0.9 for mixed commercial loads, or 1.0 for purely resistive loads.
Multiply: Watts = kVA × 1,000 × PF. For a 20 kVA generator serving loads at 0.85 PF: W = 20 × 1,000 × 0.85 = 17,000 watts (17 kW).
For sizing equipment, add 20–25% headroom above the calculated watts. This ensures the system handles load fluctuations, starting surges, and future expansion without overloading.
A 100 kVA generator does not deliver 100 kW unless the power factor is 1.0. At 0.8 PF, it only supplies 80 kW. Accurate conversion prevents generator overloading and ensures reliable backup power.
UPS units are rated in kVA. Without converting to watts using the actual load power factor, you may select a UPS that cannot sustain the real power demanded by critical equipment.
Many utilities impose penalties for low power factor because it increases current draw without proportional real power usage. Understanding kVA vs. watts helps optimize power factor correction investments.
| Load Type | Typical Power Factor | 10 kVA → Watts | Notes |
|---|---|---|---|
| Resistive Heaters | 0.95–1.0 | 9,500–10,000 W | Nearly unity PF |
| Incandescent Lighting | 1.0 | 10,000 W | Purely resistive |
| LED Lighting (with driver) | 0.90–0.99 | 9,000–9,900 W | Varies by driver quality |
| Mixed Commercial Load | 0.85–0.90 | 8,500–9,000 W | Office buildings typical |
| Induction Motors (full load) | 0.80–0.90 | 8,000–9,000 W | PF drops at partial load |
| Induction Motors (no load) | 0.15–0.30 | 1,500–3,000 W | Very poor PF when idle |
| Welding Equipment | 0.50–0.70 | 5,000–7,000 W | Highly variable |
Power factors are typical ranges. Always measure or use manufacturer data for accurate calculations.
The power triangle is a right triangle that visually represents the relationship between apparent power (kVA), real power (kW), and reactive power (kVAR). The hypotenuse is apparent power (kVA), the adjacent side is real power (kW), and the opposite side is reactive power (kVAR). The angle between kW and kVA is the power factor angle, and cos(θ) = PF. The formula kVA² = kW² + kVAR² allows you to calculate any component if you know the other two.
Transformers are rated in kVA because their losses (copper and core losses) depend on the current and voltage flowing through them, regardless of the load power factor. A transformer rated at 50 kVA can handle 50 kVA of apparent power whether the power factor is 0.5 or 1.0. The real power delivered depends entirely on the load connected to the transformer.
Power factor correction uses capacitor banks to offset the reactive power drawn by inductive loads. By adding capacitors, the reactive component decreases, bringing the power factor closer to 1.0. This reduces the apparent power (kVA) needed for the same real power (kW), lowering current draw, reducing I²R losses, and potentially avoiding utility power factor penalties.
First, calculate the total real power (kW) of all loads. Then divide by the expected power factor to get the required kVA: kVA = kW ÷ PF. Add 20–25% for motor starting surges and future expansion. For example, 40 kW of loads at 0.85 PF requires 40 ÷ 0.85 = 47.1 kVA, so a 60 kVA generator would be appropriate.
Many commercial and industrial utility tariffs include power factor penalties for facilities with PF below 0.90 or 0.85. The penalty may appear as a demand charge adjustment, a reactive power charge (per kVAR), or a multiplier applied to the demand charge. Improving your power factor through correction capacitors typically pays for itself within 1–3 years through reduced utility costs.