Watt Calculator
120 volts and 2 amps produce 240 watts of power with 60 ohms of resistance. This watt calculator uses Ohm's law and the power law to find watts, volts, amps, and ohms from any two known values.
Quick answer
Power in watts equals voltage times current (P = V x I).
What this tells you
- •Power in watts equals voltage times current (P = V x I).
- •When voltage and resistance are known, power equals voltage squared divided by resistance.
- •When current and resistance are known, power equals current squared times resistance.
How to Use
- 1Choose which two values you know: volts and amps, volts and ohms, or amps and ohms.
- 2Enter the two known values.
- 3Click Calculate to get the power in watts and the two missing values.
How It Works
Formula
P = V x I, V = I x R, P = I^2 x R, P = V^2 / ROhm's law (V = I x R) and the power law (P = V x I) combine to give three ways to compute power. From voltage and current: P = V x I. From voltage and resistance: P = V squared divided by R. From current and resistance: P = I squared times R.
Calculation note: values are processed in the order shown above, using the current input units.
Worked Examples
Volts and amps known
Power = 120V x 2A = 240 watts. Resistance = 120V / 2A = 60 ohms.
Volts and ohms known
Power = 120 x 120 / 60 = 240 watts. Current = 120V / 60ohms = 2 amps.
Amps and ohms known
Power = 2 x 2 x 60 = 240 watts. Voltage = 2A x 60ohms = 120 volts.
Common Voltages and Their Power at 1 Amp
Power in watts for common voltages when the current is 1 amp.
| Voltage | Current | Power |
|---|---|---|
| 5 V (USB) | 1 A | 5 W |
| 12 V (car) | 1 A | 12 W |
| 120 V (US outlet) | 1 A | 120 W |
| 230 V (EU outlet) | 1 A | 230 W |
Common mistakes
- Mixing AC and DC formulas. This calculator uses DC formulas. AC circuits with reactance need impedance instead of resistance.
- Using milliamps instead of amps. Convert milliamps to amps by dividing by 1,000 before entering.
- Confusing power in watts with energy in watt-hours. Watts measure instantaneous power, not energy over time.