AC Wattage Calculator – Real, Apparent & Reactive Power
Calculate real power, apparent power, and reactive power in AC electrical systems
Enter voltage (V), current (A), and power factor to instantly compute all three forms of AC power — real, apparent, and reactive — along with the phase angle.
AC Wattage Calculator – Real, Apparent & Reactive Power
Calculate real power, apparent power, and reactive power in AC electrical systems
About the AC Wattage Calculator
Alternating current (AC) power is the form of electrical energy used in virtually every home, office, and industrial facility worldwide. Unlike direct current (DC), where voltage and current maintain constant polarity, AC voltage and current continuously oscillate in a sinusoidal waveform. This oscillation introduces a phase relationship between voltage and current that gives rise to three distinct forms of power: real power, apparent power, and reactive power.
Real power (P), measured in watts (W), is the power that actually does useful work — running a motor, heating a room, or illuminating a space. It is calculated as P = V × I × cos(φ), where V is the root-mean-square (RMS) voltage, I is the RMS current, and cos(φ) is the power factor. The power factor represents the cosine of the phase angle between the voltage and current waveforms.
Apparent power (S), measured in volt-amperes (VA), is simply the product of RMS voltage and RMS current: S = V × I. It represents the total electrical load that the circuit conductors, transformers, and generators must be designed to handle, regardless of how efficiently that current does useful work. Apparent power is the key parameter for sizing electrical infrastructure.
Reactive power (Q), measured in volt-amperes reactive (VAR), is the power that oscillates back and forth between the source and reactive elements — inductors (motors, transformers) or capacitors — without performing net useful work. It is given by Q = V × I × sin(φ), and the three power quantities are related by the power triangle: S² = P² + Q². A high reactive power component lowers the power factor and causes higher currents in the distribution network.
The power factor (cos φ) is one of the most important parameters in AC circuit design. A power factor of 1.0 (purely resistive load) means every ampere drawn from the supply contributes to useful work. As the power factor falls below 1.0 due to inductive or capacitive loads, more current must flow through conductors to deliver the same amount of real power. This extra current causes additional resistive losses, increased voltage drop, and higher electricity costs — which is why large commercial and industrial users invest in power factor correction equipment such as capacitor banks.
This AC Wattage Calculator is designed for electricians, engineers, students, and homeowners who need to quickly determine power consumption characteristics of AC loads. Enter the RMS voltage, RMS current, and power factor, and the calculator instantly provides real power in watts, apparent power in volt-amperes, reactive power in VAR, and the phase angle in degrees. Use it to audit energy consumption, size electrical equipment, estimate electricity bills, or verify nameplate data on motors and appliances.
AC Power Examples
These examples show typical AC power calculations across different load types and supply voltages.
| Circuit Parameters | Power Values | Notes |
|---|---|---|
| V = 120 V, I = 5 A, PF = 1.0 | P = 600 W, S = 600 VA, Q = 0 VAR | Purely resistive household load such as an incandescent light bulb or electric heater. Power factor = 1 means all supplied power does useful work. |
| V = 220 V, I = 10 A, PF = 0.85 | P = 1870 W, S = 2200 VA, Q = 1159 VAR | Typical inductive motor load on a 220 V European supply. The reactive power represents energy cycling between source and motor windings without doing useful work. |
| V = 240 V, I = 3 A, PF = 0.92 | P = 662.4 W, S = 720 VA, Q = 281.5 VAR | Capacitive load with a leading power factor of 0.92 on a 240 V UK supply. Capacitive loads generate reactive power rather than absorbing it. |
| V = 380 V, I = 25 A, PF = 0.78 | P = 7410 W, S = 9500 VA, Q = 5945 VAR | Heavy industrial equipment on 380 V three-phase equivalent single-phase. Q = S × sin(φ) = 9500 × sin(acos(0.78)) ≈ 5945 VAR. The low power factor of 0.78 means apparent power greatly exceeds real power. |
How to use the AC Wattage Calculator
- Enter the RMS voltage in volts — for household circuits this is typically 120 V (North America) or 230–240 V (Europe/UK).
- Enter the RMS current in amperes. Use a clamp meter or check the device nameplate for rated current.
- Enter the power factor (cos φ) — a value between 0 and 1. Resistive loads (heaters, incandescent bulbs) have PF = 1. Electric motors typically range from 0.7 to 0.95.
- Optionally enter the frequency in hertz (50 Hz or 60 Hz) for reference; it does not affect the power calculations.
- Click Calculate Power to display real power (W), apparent power (VA), reactive power (VAR), and the phase angle in degrees.
AC Wattage Calculator FAQ
What is the difference between real power, apparent power, and reactive power?
Real power (P, measured in watts) is the power actually consumed and converted to useful work such as heat, light, or mechanical motion. Apparent power (S, in volt-amperes) is the product of RMS voltage and RMS current regardless of phase. Reactive power (Q, in VAR) is power that oscillates between the source and inductive or capacitive loads without doing net useful work. They are related by the power triangle: S² = P² + Q².
What is power factor and why is it important?
Power factor (PF = cos φ) is the ratio of real power to apparent power, ranging from 0 to 1. A PF of 1 means the current and voltage are perfectly in phase and all drawn current does useful work. Lower power factors cause higher currents for the same useful power output, increasing conductor losses and electricity costs. Utilities often charge industrial consumers a penalty for power factors below 0.9 or 0.95, making power factor correction economically important.
How do I calculate real power (watts) in an AC circuit?
Real power P = V × I × cos(φ), where V is the RMS voltage in volts, I is the RMS current in amperes, and cos(φ) is the power factor. For a purely resistive load (PF = 1), real power equals apparent power. For inductive or capacitive loads with a power factor less than 1, real power is always less than apparent power. This calculator performs that multiplication automatically once you enter the three inputs.
What is reactive power and how is it calculated?
Reactive power Q = V × I × sin(φ), where sin(φ) = √(1 − cos²φ). It represents energy stored and released by inductors (motors, transformers) or capacitors each half-cycle. While reactive power does no net useful work, it must still be supplied by the generator or utility, increasing cable sizing and transformer ratings. Q is measured in volt-amperes reactive (VAR).
What typical power factors do different electrical loads have?
Resistive loads such as electric heaters, incandescent bulbs, and toasters have a power factor of 1.0. Induction motors have a power factor of 0.7 to 0.9 depending on load; lightly loaded motors have lower power factors. Fluorescent and LED lighting with electronic ballasts typically have power factors of 0.85 to 0.95. Variable-frequency drives and switched-mode power supplies generally have PF of 0.95 to 0.99 with active power factor correction.
What is the formula for apparent power and when do I use it?
Apparent power S = V × I (in volt-amperes, VA). It represents the total capacity required from the power supply or generator regardless of how efficiently that power is used. Apparent power is used when sizing transformers, generators, UPS systems, and circuit breakers, because these devices must handle the full current draw irrespective of power factor. Real power tells you the useful work; apparent power tells you the electrical infrastructure capacity needed.