Watt Calculator
Calculate electrical power in watts from voltage and current
Conversions
The P = V × I Power Formula Explained
The watt calculator uses the fundamental power equation P = V × I, where P is power in watts, V is voltage in volts, and I is current in amperes. This relationship, derived from Ohm's Law, forms the basis of all electrical power calculations. James Watt, the Scottish inventor, gave his name to this unit of power measurement.
One watt equals one joule of energy transferred per second. When you multiply voltage (electrical pressure) by current (electron flow rate), you get the rate of energy consumption or production. A standard US outlet at 120V supplying 10A delivers 1,200 watts of power.
Calculating Watts from Volts and Amps
Simply multiply voltage by current to find watts. A 240V electric dryer drawing 20A uses 240 × 20 = 4,800 watts. This calculation helps determine if your circuit can handle a device - a 15A circuit at 120V provides maximum 1,800 watts before tripping the breaker.
Finding Amps When You Know Watts
Rearrange the formula to I = P / V. A 1,500W space heater on a 120V circuit draws 1,500 / 120 = 12.5 amps. This helps you determine proper wire gauge and breaker size for electrical installations.
Determining Voltage from Power and Current
Use V = P / I when you know power and current. If a motor consumes 746 watts at 6.2 amps, it operates at 746 / 6.2 = 120 volts. This verification helps troubleshoot electrical systems and confirm proper operation.
What is the Difference Between Watts and Kilowatts?
Kilowatts (kW) equal 1,000 watts. Utility companies bill in kilowatt-hours (kWh) because household consumption adds up quickly. Running a 100W light bulb for 10 hours uses 1 kWh. At $0.12 per kWh, that costs about 1.2 cents per hour or 12 cents for the full 10 hours.
Converting Watts to Kilowatts
Divide watts by 1,000 to get kilowatts. A 2,400W air conditioner equals 2.4 kW. Running it 8 hours daily consumes 19.2 kWh, costing roughly $2.30 per day at average electricity rates. Use our Ohms Law calculator for related electrical calculations.
Megawatts for Large-Scale Power
One megawatt (MW) equals 1,000,000 watts or 1,000 kW. Power plants measure output in megawatts - a typical natural gas plant produces 500-1,000 MW. A single MW can power approximately 750-1,000 average homes depending on climate and usage patterns.
How Do You Convert Watts to Horsepower?
One mechanical horsepower equals 746 watts. Divide watts by 746 to convert to horsepower. A 3,730W motor produces 3,730 / 746 = 5 horsepower. This conversion matters when comparing electric motors to gas engines or selecting replacement equipment.
Electric Motor Horsepower Ratings
Electric motors list both wattage and horsepower. A 1 HP motor draws about 746W at full load, but startup current can be 3-7 times higher. Always size circuits for startup surge, not just running watts. A 1 HP motor may need a 20A circuit despite running at only 6-7 amps.
BTU Conversion for Heating and Cooling
One watt equals 3.412 BTU per hour. Air conditioners rate cooling capacity in BTU/hr - a 12,000 BTU window unit consumes about 3,500 watts (12,000 / 3.412). This helps compare energy efficiency between different HVAC systems. Check our power calculator for physics applications.
Common Appliance Wattage Reference Table
| Appliance | Typical Watts | Amps at 120V | Monthly Cost* |
|---|---|---|---|
| LED Light Bulb | 10W | 0.08A | $0.36 |
| Laptop Computer | 50W | 0.42A | $1.80 |
| Desktop Computer | 200W | 1.67A | $7.20 |
| Refrigerator | 150W | 1.25A | $5.40 |
| Microwave Oven | 1,000W | 8.33A | $3.60 |
| Space Heater | 1,500W | 12.5A | $54.00 |
| Hair Dryer | 1,800W | 15A | $1.30 |
| Electric Dryer (240V) | 5,000W | 21A | $18.00 |
*Based on $0.12/kWh and typical monthly usage hours
Sizing Generators and Solar Panels by Wattage
Add up the wattage of all devices you need to run simultaneously to size a generator. A refrigerator (150W), lights (100W), and phone chargers (25W) need at least 275W continuous, but add 25% safety margin for 350W minimum. Account for startup surge on motors - refrigerators may need 3x running watts to start.
Solar Panel Output Calculations
A 300W solar panel produces 300 watts under ideal conditions (full sun, cool temperature). In reality, expect 70-80% of rated output. With 5 peak sun hours daily, one 300W panel generates about 1.05-1.2 kWh per day. Size your system based on daily kWh needs divided by expected production.
Circuit Breaker and Wire Sizing
Never load a circuit beyond 80% of its rated capacity for continuous loads. A 15A circuit at 120V provides 1,800W maximum, but limit continuous loads to 1,440W. For a 1,500W space heater, use a 20A circuit (2,400W max, 1,920W continuous). Use our kinetic energy calculator for mechanical power problems.
Last Updated: January 2026 | Reviewed for accuracy