Watt Calculator
Calculate electrical power in watts from voltage, current, and resistance using P=VI, P=I²R, and P=V²/R.
Calculating Electrical Power in Watts
Electrical power measures how fast energy is transferred in a circuit. Given two of voltage, current, and resistance, you can calculate power in watts using Ohm's law relationships.
Formulas
$$P = V \cdot I$$ $$P = \frac{V^2}{R}$$ $$P = I^2 \cdot R$$\(P\) is power in watts, \(V\) is voltage in volts, \(I\) is current in amps, and \(R\) is resistance in ohms. These three forms are equivalent when Ohm's law (\(V = IR\)) holds.
Practical Notes
A 60 W light bulb at 120 V draws 0.5 A. Power ratings on appliances tell you energy consumption rate. For AC circuits with reactive loads, use apparent power (VA) and power factor for true wattage.
Related tools: Power Dissipation Calculator and Ohm's Law Calculator.
Frequently Asked Questions
What is a watt?
One watt equals one joule per second. It measures the rate of energy transfer, how fast work is done or heat is produced electrically.
Which formula should I use?
Use P = VI when you know voltage and current. Use P = V²/R when you know voltage and resistance. Use P = I²R when you know current and resistance.
How do I convert watts to kilowatts?
Divide by 1000. For example, 1500 W = 1.5 kW. Utility bills often use kilowatt-hours (kWh) for energy consumed over time.
Is power the same as energy?
No. Power (watts) is the rate. Energy (joules or kWh) is power multiplied by time. A 100 W bulb running for 10 hours uses 1 kWh of energy.
Does this work for AC circuits?
These formulas apply to DC and resistive AC loads. For AC with inductors or capacitors, account for power factor to get real power in watts.
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