Power Factor Calculator
Calculate power factor, real power, reactive power, and apparent power for AC circuits.
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How to Use This Calculator
Enter the real power in kilowatts as measured by a watt meter or read from equipment nameplates. Enter the apparent power in kVA as calculated from voltage and current measurements, or enter the voltage and current directly and the calculator will compute apparent power. For three phase systems, select three phase and the calculator uses the formula kVA = V x I x 1.732 / 1000. The calculator displays the power factor as a decimal and percentage, the reactive power in kVAR, and the phase angle in degrees. For example, if a motor draws 460V at 25A on a three phase circuit and consumes 15 kW, the apparent power is 460 x 25 x 1.732 / 1000 = 19.9 kVA. The power factor is 15 / 19.9 = 0.754 or 75.4%, and the reactive power is 13.2 kVAR.
Understanding the Concept
Power factor measures how effectively electrical power is being used. A power factor of 1.0 (unity) means all the power delivered is doing useful work. A power factor less than 1.0 means some of the current flowing through the system is reactive current that does no useful work but still heats conductors and loads transformers. Inductive loads such as motors, transformers, and fluorescent ballasts are the primary cause of low power factor in commercial and industrial facilities. Low power factor has real costs: utility companies charge power factor penalties (often below 0.90), conductors must be larger to carry the extra reactive current, transformer and generator capacity is wasted, and voltage drop increases. Power factor correction is achieved by adding capacitors that supply reactive power locally, reducing the reactive current drawn from the utility. The corrected power factor reduces demand charges, frees up transformer capacity, and improves voltage regulation throughout the facility.
The Formula Explained
Power factor is calculated as PF = Real Power (kW) / Apparent Power (kVA). Apparent power is the vector sum of real power and reactive power, related by the power triangle: kVA squared = kW squared + kVAR squared. The phase angle theta between voltage and current is found from PF = cos(theta), so theta = arccos(PF). Reactive power is calculated as kVAR = kVA x sin(theta), or equivalently kVAR = square root of (kVA squared minus kW squared). For power factor correction, the required capacitor kVAR is kVAR_correction = kW x (tan(theta_original) minus tan(theta_target)). For example, to correct a 100 kW load from 0.80 PF (theta = 36.87 degrees) to 0.95 PF (theta = 18.19 degrees), the required capacitors are 100 x (0.750 minus 0.329) = 42.1 kVAR. NEC 460 governs the installation of power factor correction capacitors.
Frequently Asked Questions
What is a good power factor for a commercial building?
A power factor of 0.95 or higher is considered good for commercial buildings. Most utilities begin charging power factor penalties when the factor drops below 0.90, and some penalize below 0.85. Industrial facilities with many motors often have uncorrected power factors of 0.70 to 0.85. Installing power factor correction capacitors to bring the facility above 0.95 typically pays for itself within one to two years through reduced utility demand charges and lower current flow in the electrical distribution system.
What causes low power factor?
Inductive loads are the primary cause of low power factor. Electric motors, especially when running at partial load, draw significant reactive current. Transformers, fluorescent lighting ballasts, and induction furnaces also contribute. Lightly loaded motors are particularly problematic because the reactive current remains relatively constant even as the real power drops. A motor at 50% load might have a power factor of 0.65, compared to 0.85 at full load. Variable frequency drives and modern LED lighting have improved power factor in many facilities, but motor loads remain the dominant factor.
How do power factor correction capacitors work?
Capacitors supply reactive power (kVAR) locally at the load, reducing the reactive current that must flow from the utility through the entire distribution system. Since capacitive reactive power is opposite in phase to inductive reactive power, they cancel each other out. The net result is that the transformer and feeder conductors carry less total current for the same real power delivered. Capacitors can be installed at individual motors, at distribution panels, or at the main switchgear. Individual motor correction is most effective but more expensive to install. NEC 460.8 requires a disconnect for each capacitor installation.
What is the difference between leading and lagging power factor?
Lagging power factor occurs when the current waveform lags behind the voltage waveform, caused by inductive loads like motors. This is the most common condition in buildings. Leading power factor occurs when the current leads the voltage, caused by capacitive loads or overcorrection with too many capacitors. Overcorrection to a leading power factor can cause voltage rise, resonance problems, and equipment damage. Power factor correction should target 0.95 to 0.98 lagging rather than unity to avoid the risk of leading power factor during light load periods.
Do utility companies charge for poor power factor?
Yes. Most commercial and industrial utility tariffs include a power factor penalty or incentive. Common structures include a direct kVAR demand charge, a billed demand adjustment that increases the demand charge when PF falls below a threshold (typically 0.90), or a multiplier on the total bill. The penalty can add 10% to 30% to monthly electricity costs for facilities with power factors below 0.80. Some utilities offer rebates for installing power factor correction equipment. Residential customers are generally not subject to power factor charges.