Electrical power, measured in watts, is the rate at which energy is transferred in an electric circuit. Power can be calculated using the Joule’s law equation: “Power = Voltage x Current.” Voltage (measured in volts) is the difference of electric potentials that is a driving force of the electric current (measured in amperes). Combining the Joule’s and Ohm's laws, it is also possible to calculate power using electrical resistance (in Ohms).

Multiply voltage and current to calculate electrical power in watts. For example, if voltage is 25 volts and current is 1.6 amperes, then power equals to 40 watts (25 volts x 1.6 amperes).

Divide the electrical power by voltage to calculate the current in the circuit. For instance, power is 75 watts and voltage is 50 volts. Current = 75 watts / 50 volts = 1.5 amperes.

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Divide the electrical power by the current to calculate the voltage. For instance, power is 100 watts and current is 0.9 amperes. Voltage = 100 watts / 0.9 amperes = 111.11 volts.

Raise voltage to the power of 2 and then divide by the resistance to calculate the electrical power. For example, the voltage is 127 volts and the resistance is 200 ohms. Power = 127 volts x 127 volts / 200 ohms = 80.65 watts.

Raise the current to the power of 2 and then multiply by the resistance to calculate the electrical power. For example, if the current is 1.5 amperes and the resistance is 250 ohms, then the power is 562.5 watts (1.5 amperes x 1.5 amperes x 250 ohms).