Power Calculator
Calculate electrical power in watts from voltage and current. Solve for power, voltage, current, or resistance using Ohm's law formulas.
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Calculated Values
Common Electrical Appliance Power Ratings
Reference table for typical power consumption of household appliances. Actual values vary by model and efficiency rating.
| Appliance | Power (Watts) | Current at 120V |
|---|---|---|
| LED Light Bulb | 10 W | 0.08 A |
| Laptop Computer | 50 W | 0.42 A |
| Desktop Computer | 200-500 W | 1.7-4.2 A |
| Refrigerator | 100-400 W | 0.8-3.3 A |
| Microwave Oven | 1,000 W | 8.33 A |
| Hair Dryer | 1,500 W | 12.5 A |
| Electric Heater | 1,500 W | 12.5 A |
| Air Conditioner (Window) | 500-1,500 W | 4.2-12.5 A |
The Electrical Power Formula P = V × I
Electrical power measures the rate at which electrical energy is transferred in a circuit. Power in watts equals voltage in volts multiplied by current in amperes. This fundamental relationship, P = V × I, forms the basis for all electrical power calculations in DC (direct current) circuits.
Power Formulas:
- P = V × I (Power = Voltage × Current)
- P = I² × R (Power = Current² × Resistance)
- P = V² / R (Power = Voltage² / Resistance)
Calculating Power from Voltage and Current
The simplest power calculation multiplies voltage by current. A 120V outlet powering a device drawing 10 amps delivers 1,200 watts (120 × 10 = 1,200). This same formula works in reverse: a 60-watt light bulb on a 120V circuit draws 0.5 amps (60 ÷ 120 = 0.5).
How Do Watts, Volts, and Amps Relate?
These three units describe different aspects of electricity. Volts measure electrical pressure (potential difference), amps measure electrical flow (current), and watts measure power (the rate of energy transfer). A common analogy compares electricity to water: voltage is water pressure, current is flow rate, and power is the work the water can perform.
Knowing any two values allows calculating the third. If you know a circuit has 240 volts and needs 2,400 watts, the current must be 10 amps (2,400 ÷ 240 = 10). This calculation is essential for sizing circuit breakers, selecting wire gauges, and ensuring electrical safety.
Power Factor in AC Circuits
AC (alternating current) circuits introduce an additional consideration: power factor. Power factor is a ratio between real power (watts) and apparent power (volt-amps), ranging from 0 to 1. Purely resistive loads like heaters have a power factor of 1, while motors and other inductive loads typically have power factors of 0.7-0.9. The formula becomes P = V × I × PF for AC circuits.
Power Unit Conversions
Different industries use different power units. Electrical utilities bill in kilowatt-hours (kWh), automotive uses horsepower (hp), and HVAC systems rate in BTU/hour. Key conversions: 1 kilowatt = 1,000 watts, 1 horsepower = 746 watts, and 1 watt = 3.412 BTU/hour. Our calculator automatically converts between all these units.
For related electrical calculations, try our Ohm's law calculator for resistance calculations, watt calculator for power conversions, or voltage calculator for electrical potential.
Last Updated: January 2026 | Reviewed for accuracy