Apparent vs Real Power (kVA / kW)
Definition
Real power, in kilowatts (kW), is the power converted into light, sound or heat, while apparent power, in kilovolt-amperes (kVA), is the product of the RMS voltage and current that the source must supply. The power factor (PF) is the ratio kW / kVA. It falls below 1 when the current is out of phase with the voltage, as with motors, or when the current waveform is distorted, as with the rectifier input of an electronic power supply without power factor correction. Generators, transformers and distribution equipment are limited by current and are therefore rated in kVA. Generating sets are commonly rated at a power factor of 0.8, so a 100 kVA set supplies at most 80 kW.
In the field
Generators and distribution are sized in kVA, from the total real power divided by the lowest expected power factor. A 40 kW LED rig at a power factor of 0.9 requires about 44 kVA, so a 40 kVA generator is already overloaded. Harmonic currents from electronic loads also heat generator windings, and generator manufacturers publish sizing guidance for non-linear loads.
Related lessons
Sources
- IEC 60050-131 (Electropedia), IEV 131-11-46 : facteur de puissance · electropedia.org
- Caterpillar, Demystifying Generator Set Ratings : classes de puissance des groupes électrogènes comparées à l'ISO 8528-1 ; groupes triphasés cotés à PF 0,8 · cat.com
- Leonardo Energy, How should I size a diesel generator unit? Classes de puissance ISO 8528-1 (continue, principale, secours) et taux de charge · help.leonardo-energy.org
- STAMFORD (Cummins), AGN 025 Non Linear Loads : les charges non linéaires imposent un alternateur dimensionné en conséquence (impédance de source, distorsion harmonique) · stamford-avk.com
- Calcul : 40 kW / 0,9 = 44,4 kVA ; 100 kVA × 0,8 = 80 kW