s Law and Ohm's Law combined."> Power Calculator | BulkCalculator
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Power Calculator

Calculate electrical power (watts)

Enter any two values to calculate the others:

V
A
Ω
W
Result
Enter 2 values

Power Formulas

BasicP = V × I
With RP = I²R = V²/R

Units

1 kW = 1000 W
1 MW = 1,000,000 W
1 HP ≈ 746 W
1 kWh = 1 kW for 1 hour

Related

Ohm's LawWatts to AmpskW to Amps

Understanding Electrical Power

Electrical power is the rate at which electrical energy is transferred or consumed. It's measured in watts (W), named after James Watt. Power tells you how fast energy is being used.

Power Formulas

P = V × I - Power equals voltage times current (Watt's Law)

P = I²R - Power equals current squared times resistance

P = V²/R - Power equals voltage squared divided by resistance

Power vs Energy

Power (watts) is the rate of energy use. Energy (watt-hours) is power multiplied by time. Your electricity bill charges for energy (kWh), not power. A 100W bulb running for 10 hours uses 1 kWh.

AC Power

In AC circuits with reactive loads, real power P = V × I × cos(θ) where cos(θ) is the power factor. Apparent power S = V × I is measured in VA. Real power is what does useful work.

Frequently Asked Questions

The basic power formula is P = V × I (watts = volts × amps). Combined with Ohm's Law, you also get P = I²R and P = V²/R. These let you calculate power knowing any two of: voltage, current, or resistance.

Simply multiply: Watts = Volts × Amps. Example: A 120V outlet supplying 10 amps provides 120 × 10 = 1200 watts of power. This works for DC and resistive AC loads.

Power is the rate of energy use (watts). Energy is power × time (watt-hours or joules). Think of power as speed and energy as distance. A 1000W appliance running 1 hour uses 1000Wh = 1kWh of energy.

Power factor (PF) is the ratio of real power to apparent power, ranging from 0 to 1. Real power P = V × I × PF. Resistive loads (heaters, bulbs) have PF ≈ 1. Motors/inductors have PF ≈ 0.8-0.95.

1 mechanical horsepower = 746 watts. 1 metric horsepower = 735.5 watts. To convert HP to watts, multiply by 746. A 5 HP motor uses about 3730 watts at full load (assuming 100% efficiency).

P = I²R shows power loss is proportional to current squared. Doubling current quadruples power loss. This is why high-voltage transmission lines are used - same power with lower current means dramatically less loss.