Calculate apparent power (VA), real power (W), reactive power (VAR), and power factor for AC circuits. Size transformers and generators accurately.
The Apparent Power Calculator computes the relationship between apparent power (VA), real power (W), reactive power (VAR), and power factor in AC electrical circuits. Understanding these relationships is essential for properly sizing transformers, generators, UPS systems, and electrical distribution equipment.
In AC circuits, the power consumed by a load isn't simply voltage times current. Inductive and capacitive loads cause current to lag or lead voltage, creating reactive power that does no useful work but still flows through conductors and transformers. Apparent power (measured in volt-amperes) represents the total power the source must deliver, while real power (watts) is the actual work performed.
Enter any two known values — voltage and current, real power and power factor, or apparent power and power factor — to compute the complete power triangle including all three power components and the phase angle. It gives you a quick check before you choose a transformer or generator size.
Use this calculator when you need to size AC equipment from the full power triangle instead of treating watts and amps as interchangeable. It is useful for transformer sizing, generator planning, and checking whether a load will stress a circuit because of poor power factor. That helps you stay within the actual VA limit instead of just the watt rating.
Apparent Power S (VA) = V × I. Real Power P (W) = S × cos(θ) = S × PF. Reactive Power Q (VAR) = S × sin(θ) = S × √(1 - PF²). Power Factor = P / S = cos(θ). Phase Angle θ = arccos(PF). Three-Phase: S = √3 × V_L × I_L.
Result: S = 4,800 VA, P = 4,080 W, Q = 2,530 VAR
S = 240 × 20 = 4,800 VA. P = 4,800 × 0.85 = 4,080 W. Q = 4,800 × sin(arccos(0.85)) = 4,800 × 0.527 = 2,530 VAR. Phase angle = 31.8°.
The power triangle is a right triangle relating the three types of AC power. The hypotenuse is apparent power S (VA), the adjacent side is real power P (W), and the opposite side is reactive power Q (VAR). The angle θ between S and P is the phase angle, and cos(θ) equals the power factor.
When PF = 1, the triangle collapses to a line (S = P, Q = 0). As PF decreases, Q grows and S increases for the same P. At PF = 0.5, the source must deliver twice the apparent power to produce the same real work.
Three-phase systems are standard in commercial and industrial settings. For balanced three-phase loads: S = √3 × V_line × I_line. The √3 factor (≈1.732) appears because three phases share the load. Three-phase power is more efficient for transmission and provides constant instantaneous power delivery, unlike single-phase which pulsates at twice the line frequency.
Utility companies typically charge demand penalties when power factor drops below 0.85-0.90. The cost of capacitor banks pays back in 1-3 years through reduced demand charges, lower I²R losses, and freed-up system capacity. Automatic power factor correction controllers switch capacitor banks on and off to maintain target PF as loads vary throughout the day.
Watts (W) measure real power — the actual work performed. Volt-amperes (VA) measure apparent power — the total power flowing in the circuit. They differ when power factor is less than 1. VA ≥ W always. For purely resistive loads (PF=1), VA = W.
Low power factor means more current flows for the same real work, requiring larger wires, transformers, and generators. Utilities may charge penalties for power factors below 0.85-0.90. Poor PF wastes system capacity and increases losses.
Add capacitor banks to offset inductive loads (most common). Use synchronous condensers for large facilities. Install active power factor correction (PFC) on electronic equipment. Target PF of 0.95+ for optimal efficiency.
Resistive heaters: PF = 1.0. LED lights with PFC: 0.90-0.99. Motors at full load: 0.85-0.90. Motors at light load: 0.50-0.70. Welders: 0.50-0.60. Computers with PFC: 0.95-0.99.
Transformers are rated in kVA (apparent power), not kW. Divide your real power (kW) by the power factor to get required kVA. Example: 100 kW load at PF = 0.8 needs 100/0.8 = 125 kVA transformer.
Lagging PF means current lags voltage — caused by inductive loads (motors, transformers). Leading PF means current leads voltage — caused by capacitive loads (power factor correction caps, lightly loaded cables). Most industrial loads are lagging.