How to Calculate Power From Voltage

Voltage (measured in volts) is a difference of electric potentials that is a driving force of the electric current. The electric power (in watts) is a rate at which the energy is transferred in the electric circuit. According to the Joule's law, the power is a function of the voltage and the electric current (in amperes). Also, it can be calculated using the voltage and the electrical resistance (in ohms).

Things You'll Need

• Calculator

Instructions

1. Obtain either voltage and current values or voltage and resistance ones to calculate the electric power. As an example, consider two sets of values: voltage, 200 volts; current, 1.4 amperes; and voltage, 127 volts; resistance, 150 ohms.

2. Calculate the power from the voltage and the current using Joule's law:

   Power = Voltage x Current

   In our example:

   Power = 200 volts x 1.4 amperes = 280 watts.

3. Calculate the power from voltage and the resistance using Joule's and Ohm's laws. Ohm's law states:

   Current = Voltage / Resistance

   Combining it with the Joule's law (Step 2), you get:

   Power = (Voltage x Voltage) / Resistance

   In our example:

   Power = (127 volts x 127 volts) / 150 ohms = 107.53 watts.

References

• Physics; M. Alonso and E. Finn; Jun. 10, 1992