What is the difference between watts and horsepower?

One mechanical horsepower equals 745.7 watts. The term originated from James Watt comparing steam engine output to draft horses. A typical car engine produces 100-300 hp (75-225 kW).

What is a kilowatt-hour?

A kilowatt-hour (kWh) is a unit of energy equal to using 1000 watts for one hour (3,600,000 joules). It is the standard billing unit for electricity. Running a 100W light bulb for 10 hours uses 1 kWh.

How is electrical power calculated?

For DC circuits: P = V × I. For AC circuits: P = V × I × cos(phi), where cos(phi) is the power factor accounting for the phase difference between voltage and current in reactive loads.

What is peak power versus average power?

Peak power is the maximum instantaneous power output, while average power is the total energy over the total time. A camera flash may have peak power of 1000W but average power of only 1W because it fires briefly.

How efficient are common energy conversions?

Electric motors: 85-95% efficient. Internal combustion engines: 20-40%. Solar panels: 15-22%. LED lights: 40-50% (light output). Incandescent bulbs: 2-5%. Efficiency is the ratio of useful power output to total power input.

Power Calculator

Calculate electrical power (P = V x I)

Power Calculator

Enter voltage and current

Voltage (V)

Current (A)

Formula

P = V x I

What Is Electrical Power?

Electrical power is the rate at which electrical energy is transferred or converted. Measured in watts (W), it represents how fast a device consumes or produces energy. A 100-watt light bulb, for example, converts 100 joules of electrical energy into heat and light every second.

Power calculations are fundamental to electrical engineering and everyday life. Whether you are sizing a generator, calculating energy costs, selecting a fuse, or designing a circuit, understanding P = V × I is essential. Combined with Ohm's Law, you can find any of the four quantities — power, voltage, current, or resistance — if you know any two.

How to Use This Calculator

Select which quantity you want to solve for: Power (P), Voltage (V), Current (I), or Resistance (R).
Enter the two known values with their units (watts, volts, amps, or ohms).
Click Calculate to see the result instantly.
Use Reset to clear the fields and start a new calculation.

Formula & Explanation

P = V × I
P = I² × R
P = V² / R
V = P / I
I = P / V
R = V² / P

P = Power in watts (W), V = Voltage in volts (V), I = Current in amperes (A), R = Resistance in ohms (Ω). All six forms are algebraic equivalents derived from P = V × I and V = I × R.

Worked Examples

Example 1 — Light Bulb Power

A 120 V circuit carries 0.833 A through a bulb. P = V × I = 120 × 0.833 ≈ 100 W.

Example 2 — Resistor Dissipation

A 10 Ω resistor carries 2 A. P = I² × R = 4 × 10 = 40 W of heat dissipated.

Example 3 — Appliance Current Draw

A 1,200 W microwave runs on 120 V. I = P / V = 1200 / 120 = 10 A drawn from the outlet.

Frequently Asked Questions

What is the difference between watts and volt-amps?▾

Watts (W) measure real power — the energy actually consumed. Volt-amps (VA) measure apparent power, which includes reactive power in AC circuits with capacitors or inductors. For purely resistive loads, W = VA. For motors or transformers, VA > W, and the ratio is the power factor.

How do I calculate my electricity bill?▾

Energy consumed = Power (kW) × Time (hours) = kilowatt-hours (kWh). Multiply kWh by your utility's rate (e.g., $0.12/kWh) to get the cost. A 100 W bulb running 10 hours uses 1 kWh = about $0.12.

What is the difference between AC and DC power calculations?▾

For DC circuits, P = V × I exactly. For AC circuits, P = Vrms × Irms × power factor. The power factor accounts for the phase difference between voltage and current in reactive circuits.

Why do resistors have power ratings?▾

When current flows through a resistor, it dissipates power as heat (P = I² × R). If the actual power exceeds the resistor's rating (e.g., ¼ W, ½ W, 1 W), it overheats and fails. Always choose a resistor with a power rating at least 2× the calculated dissipation.

What is 3-phase power?▾

Three-phase power is used in industrial and commercial electrical systems. Total power P = √3 × VL × IL × PF, where VL is line voltage, IL is line current, and PF is power factor. Three-phase systems are more efficient for transmitting large amounts of power.