Electric Power Calculator

Calculate electrical power using P = VI, P = I²R, or P = V²/R. Also calculate energy consumption over time.

Voltage (V)

Current (A)

How to Calculate Electric Power

Step 1: Choose your calculation method based on what values you know - voltage and current, current and resistance, or voltage and resistance.

Step 2: Enter your values in the appropriate fields above.

Step 3: Click Calculate to see the power result in watts plus conversions to kilowatts, horsepower, and BTU/hour.

Understanding Electric Power Calculations

What Is Electric Power

Electric power measures how fast electrical energy transfers through a circuit. Think of it like water flowing through a pipe - voltage is the pressure pushing the water, current is how much water flows, and power is the total work the water can do. The standard unit is the watt (W), named after James Watt.

Three Ways to Calculate Power

You can find power using different combinations of electrical values:

P = V × I

Use when you know voltage and current. Most common for household appliances.

P = I² × R

Use when you know current and resistance. Common in circuit analysis.

P = V² / R

Use when you know voltage and resistance. Useful for load calculations.

Why Power Calculations Matter

Knowing how much power your devices use helps you size circuits correctly, estimate electricity costs, and avoid overloading outlets. A typical household circuit handles 15-20 amps at 120 volts, giving you 1,800-2,400 watts to work with.

Common Power Ratings Reference

Device	Typical Power (W)	Current at 120V (A)
LED light bulb	8-12 W	0.07-0.1 A
Laptop charger	45-90 W	0.4-0.75 A
Refrigerator	150-400 W	1.25-3.3 A
Microwave	800-1,200 W	6.7-10 A
Space heater	1,500 W	12.5 A
Hair dryer	1,200-1,875 W	10-15.6 A
Electric oven	2,000-5,000 W	16.7-41.7 A
Central AC (3 ton)	3,500 W	29 A

Actual power consumption varies by model and usage. Check device labels for exact ratings.

Power Consumption Examples

Daily energy use for common devices. Multiply wattage by hours of use to get watt-hours. A typical US household uses 30,000 Wh (30 kWh) per day.

Electric Power Formulas Reference

Basic Power Formula

P = V × I

Power (watts) equals voltage (volts) times current (amps). This is the most direct way to calculate power in DC circuits and AC circuits with resistive loads.

Power from Current and Resistance

P = I² × R

Substitute V = I × R (Ohm's Law) into P = V × I. This form shows that power loss in a resistor increases with the square of current - doubling the current quadruples the power dissipation.

Power from Voltage and Resistance

P = V² / R

Another Ohm's Law substitution. Useful when you know the voltage drop across a component and its resistance. Shows that for a fixed resistance, power increases with the square of voltage.

Energy Consumption

Energy = Power × Time

Energy (watt-hours) equals power (watts) times time (hours). Your electricity bill charges you per kilowatt-hour (kWh), which is 1,000 watt-hours.

Frequently Asked Questions

How do I calculate electric power?

Use P = V × I if you know voltage and current. For example, a 120V device drawing 2A uses 240W. If you know resistance instead, use P = I²R or P = V²/R depending on what values you have.

What is the difference between watts and watt-hours?

Watts measure power - how fast energy is used right now. Watt-hours measure energy - the total amount used over time. A 100W bulb running for 10 hours uses 1,000 Wh (1 kWh) of energy.

How many watts can a 15 amp outlet handle?

A 15A outlet at 120V can handle up to 1,800W (15 × 120). For continuous loads like space heaters, use 80% of that - about 1,440W. A 20A outlet handles 2,400W, or 1,920W for continuous use.

Does higher wattage always mean more electricity cost?

Higher wattage means faster energy use, but cost depends on how long you run the device. A 1,500W heater running 1 hour costs the same as a 100W bulb running 15 hours. Check your electricity rate - the US average is around 15 cents per kWh.

Can I use this calculator for AC circuits?

Yes, for resistive loads like heaters and incandescent bulbs. For motors, transformers, or electronics, you need to account for power factor. Real power (watts) = V × I × power factor. Power factor ranges from 0 to 1, with 1 being purely resistive.

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