Power Factor Calculator
Calculate power factor, reactive power, and phase angle from real and apparent power.
This tool is for informational and educational purposes only. It is not a substitute for professional financial, medical, legal, or engineering advice. See Terms of Service.
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Power factor describes how efficiently electrical power is being used in an AC circuit. A power factor of 1.0 (unity) means all power is used as real work. Lower values indicate reactive power is present.
- Enter real power (W). This is the actual power consumed by the load doing work, measured in watts. Find it on equipment specifications or measure with a power meter.
- Enter apparent power (VA). This is the total power drawn from the supply, including reactive power, measured in volt-amperes. It equals voltage times current (V × I).
- Read the results. The calculator shows power factor, reactive power in VAR (volt-ampere reactive), and the phase angle between voltage and current.
About Power Factor
Power factor = real power / apparent power = cos(φ), where φ is the phase angle between voltage and current. The reactive power (Q) forms the third side of the "power triangle": Q = √(VA² - W²). Reactive power does no useful work but must be supplied by the source and handled by the distribution system.
Industrial facilities with large motors and fluorescent lighting systems often have power factors of 0.7 to 0.85, causing utilities to charge demand penalties. Power factor correction capacitors are added in parallel with inductive loads to bring the power factor closer to unity, reducing reactive current and utility charges.
Frequently Asked Questions
What is a good power factor?
A power factor of 1.0 (unity) is ideal, meaning all power drawn from the supply does useful work. For most practical purposes, a power factor above 0.95 is considered excellent. Most utility companies require industrial customers to maintain a power factor above 0.85 to 0.90 or face demand surcharges. Modern switching power supplies and variable-frequency drives typically achieve power factors of 0.95 to 0.99 with active power factor correction (PFC).
What causes a low power factor?
Low power factor is primarily caused by inductive loads: electric motors (especially lightly loaded motors), transformers, fluorescent and HID lighting with magnetic ballasts, and older power supplies without power factor correction. These devices draw reactive (magnetizing) current that lags the voltage, increasing apparent power without doing useful work. Capacitive loads (certain power supply designs, long cable runs) can also cause poor power factor, but in the leading direction.
How is power factor related to UPS sizing?
A UPS is rated in VA (apparent power), not watts. To find the required VA, divide the total load in watts by the power factor: VA = W / PF. A 700 W load with a 0.7 power factor requires 1000 VA of UPS capacity. Modern server and computer equipment often has a power factor of 0.95 to 1.0 (due to active PFC), so a 700 W load might only need 737 VA. Always check the actual power factor of your equipment when sizing a UPS.
What is reactive power (VAR) and why does it matter?
Reactive power (measured in VAR, volt-ampere reactive) is power that oscillates between the source and energy-storing elements (inductors and capacitors) without being consumed. It does not do useful work, but it must flow through conductors and equipment, causing resistive losses and requiring oversized infrastructure. Utilities must generate and transmit this reactive power even though they cannot bill for it directly, which is why they charge industrial customers for low power factor.