Loading Calculator...
Please wait a moment
Please wait a moment
Typical values: 0.8 for motors, 0.95-1.0 for resistive loads
S(kVA) = P(W) / (1000 × PF)
Where:
Generators and transformers are rated in kVA (apparent power), not watts (real power). To properly size a generator or transformer for your load, you need to know the kVA requirement, which accounts for reactive power in the system.
| Watts | PF=0.8 | PF=0.85 | PF=0.9 | PF=0.95 | PF=1.0 |
|---|---|---|---|---|---|
| 500 W | 0.625 kVA | 0.588 kVA | 0.556 kVA | 0.526 kVA | 0.5 kVA |
| 1,000 W | 1.25 kVA | 1.176 kVA | 1.111 kVA | 1.053 kVA | 1 kVA |
| 2,000 W | 2.5 kVA | 2.353 kVA | 2.222 kVA | 2.105 kVA | 2 kVA |
| 5,000 W | 6.25 kVA | 5.882 kVA | 5.556 kVA | 5.263 kVA | 5 kVA |
| 10,000 W | 12.5 kVA | 11.76 kVA | 11.11 kVA | 10.53 kVA | 10 kVA |
| 15,000 W | 18.75 kVA | 17.65 kVA | 16.67 kVA | 15.79 kVA | 15 kVA |
| 20,000 W | 25 kVA | 23.53 kVA | 22.22 kVA | 21.05 kVA | 20 kVA |
| 50,000 W | 62.5 kVA | 58.82 kVA | 55.56 kVA | 52.63 kVA | 50 kVA |
Converting watts to kilovolt-amperes (kVA) translates real power into apparent power, which is the total electrical load that generators, transformers, and UPS systems must handle. Real power (watts) performs useful work — heating, lighting, or running motors — while apparent power (kVA) includes both the real component and reactive power caused by inductive and capacitive loads. The relationship is kVA = W ÷ (1000 × PF), where PF is the power factor representing how efficiently the load uses current. In a purely resistive circuit, kVA equals kW, but most real-world loads include motors, transformers, or switched power supplies that create a phase difference between voltage and current. This mismatch means the equipment must supply more total current than what is converted to useful work, making the kVA rating higher than the kW value.
Add up the wattage of all equipment that the generator or transformer will supply. Include motors at their full-load wattage and consider startup surge requirements, which can be 3–6 times the running wattage for motor loads.
Check equipment specifications for power factor values. Common values: resistive loads (heaters, incandescent lights) = 1.0, motors = 0.75–0.90, computers = 0.95–0.99 with PFC, fluorescent lighting = 0.90–0.95. For mixed loads, use 0.8 as a conservative estimate.
Divide the total watts by 1000 and then by the power factor: kVA = W ÷ (1000 × PF). For example, 15,000 watts at PF 0.85 = 15,000 ÷ (1000 × 0.85) = 17.65 kVA.
Add 20–25% to the calculated kVA for a safety margin. This accounts for future load growth, motor starting surges, and ensures the generator or transformer operates within its efficient range. Select the next standard kVA rating above your calculated value.
Generators are rated in kVA, not kW. An undersized generator overheats and fails prematurely. Proper watts-to-kVA conversion ensures you select a generator that handles both real and reactive power demands.
Transformer ratings are in kVA because they are limited by winding current capacity. IEEE C57.12 standards specify transformer ratings in kVA to account for the full apparent power load.
Utilities often charge industrial customers based on kVA demand. Improving power factor reduces the kVA-to-kW ratio, lowering demand charges and reducing the size of required equipment.
| kVA Rating | kW @ PF 0.8 | kW @ PF 1.0 | Typical Use |
|---|---|---|---|
| 5 kVA | 4 kW | 5 kW | Small portable generator |
| 10 kVA | 8 kW | 10 kW | Home standby generator |
| 25 kVA | 20 kW | 25 kW | Small commercial / residential transformer |
| 50 kVA | 40 kW | 50 kW | Commercial generator / pad-mount transformer |
| 100 kVA | 80 kW | 100 kW | Medium commercial building |
| 500 kVA | 400 kW | 500 kW | Large commercial / light industrial |
| 1000 kVA | 800 kW | 1000 kW | Industrial plant / data center |
kW (kilowatts) measures real or active power — the power that actually performs work such as producing heat, light, or mechanical motion. kVA (kilovolt-amperes) measures apparent power, which includes both real power and reactive power. Reactive power oscillates between the source and inductive/capacitive loads without performing useful work but still requires current to flow. The relationship is kW = kVA × PF. At unity power factor (PF = 1.0), kW equals kVA; at typical industrial power factors of 0.8, a 100 kVA transformer delivers only 80 kW of useful power.
Generators are rated in kVA because their internal components — windings, insulation, and cooling systems — are limited by the total current flowing through them, regardless of power factor. The kVA rating represents the maximum current the generator can safely supply at its rated voltage. The actual kW output depends on the connected load's power factor, which the generator manufacturer cannot predict. Per IEEE and NEMA standards, kVA is the standard rating for all rotating electrical machines and transformers.
Power factor correction capacitors supply reactive current locally, reducing the reactive power that must flow from the generator or utility transformer. Improving PF from 0.7 to 0.95 reduces kVA by 26% for the same kW load. For example, a 100 kW load at PF 0.7 requires 143 kVA, but at PF 0.95 it requires only 105 kVA. This allows existing transformers and generators to serve larger real-power loads and reduces utility demand charges per IEEE Std 1459.
Standard induction motors at full load typically have power factors between 0.80 and 0.90. At partial loads, the power factor drops significantly — a motor running at 25% load may have a PF as low as 0.50. For conservative generator sizing with mixed motor loads, use 0.8. For mostly motor loads, consider 0.75. Motor nameplate data usually lists the full-load power factor, which provides the most accurate value for calculations.
Add 20–25% beyond the calculated kVA for general applications. For loads with large motors, add 30–50% to handle starting surges (motors draw 5–8 times running current during startup). Also consider future load growth. Select the next standard kVA rating above your calculation. Running a generator at 70–80% of its rating optimizes fuel efficiency and extends equipment life compared to operating near maximum capacity.