Electrical Power Calculator

Enter any two values to calculate the other two. Leave fields empty for the values you want to find.

V
A
Ω
W

Enter exactly 2 of the 4 values. The other two will be calculated.

Voltage

—

Electrical potential

Current

—

Charge flow rate

Resistance

—

Opposition to current

Power

—

Energy transfer rate

Formulas Used

Enter two values to see the formulas applied.

Electrical Power Explained

The Power Wheel

The power wheel (or power triangle) shows all relationships between voltage (V), current (I), resistance (R), and power (P):

  • P = V × I — Power from voltage and current
  • P = I² × R — Power from current and resistance
  • P = V² / R — Power from voltage and resistance
  • V = I × R — Voltage from current and resistance
  • I = V / R — Current from voltage and resistance

AC vs DC Power

DC power is straightforward: P = V × I. What you measure is what you get.

AC power has three components:

  • Real power (W): The actual power consumed and converted to useful work
  • Reactive power (VAR): Power stored and released by inductors and capacitors
  • Apparent power (VA): The total power supplied, combining real and reactive

For purely resistive AC loads (heaters, incandescent bulbs), P = V × I works. For motors and other inductive loads, multiply by the power factor.

Watts vs Kilowatts

1 kilowatt (kW) = 1,000 watts. Electrical bills use kilowatt-hours (kWh), which measure energy consumed over time. A 1,000W device running for 1 hour uses 1 kWh. At $0.12/kWh, that costs about 12 cents.

Common Power Ratings

  • LED bulb: 8-15 W
  • Laptop: 45-100 W
  • Microwave: 600-1,200 W
  • Space heater: 1,500 W
  • Electric dryer: 4,000-5,000 W

Frequently Asked Questions

What is the difference between watts, amps, and volts?
Watts measure power (the rate of energy transfer), amps measure current (the flow of electric charge), and volts measure voltage (the electrical pressure that pushes current through a circuit). A common analogy is water flowing through a pipe: voltage is the water pressure, current is the flow rate, and power is the total work being done by the water.
How do I convert watts to amps?
To convert watts to amps, you need to know the voltage. The formula is Amps = Watts / Volts. For example, a 1,200-watt device on a 120V circuit draws 10 amps (1200 / 120 = 10). This is useful for determining if a device will trip a circuit breaker.
What is the difference between AC and DC power?
DC (direct current) flows in one constant direction, like from a battery. AC (alternating current) reverses direction many times per second (60 Hz in North America, 50 Hz in most other countries). AC is used for household power because it transmits efficiently over long distances. DC power calculations are straightforward (P = VI), while AC requires accounting for power factor when reactive loads are involved.
What is power factor and why does it matter?
Power factor is the ratio of real power (watts) to apparent power (volt-amps) in an AC circuit. It ranges from 0 to 1, where 1 means all power is doing useful work. Motors, transformers, and fluorescent lights have power factors less than 1 because they store and release energy in magnetic fields. Low power factor means you need more current (and larger wires) to deliver the same real power.

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