The complete path of the flow of electric current, usually including the source of electric energy. The intensity or pressure of the electric current is expressed as volts. The rate of flow or current in the circuit is expressed as amperes.

In a direct-current (d-c) circuit (sometimes called a continuous current circuit), the product of the volts and the amperes in the circuit represents the amount of electrical power (watts) produced. The product of the volts and the amperes in the circuit, together with the time of flow, is expressed as watt hours of energy expended in the circuit. One thousand watts are expressed as a kilowatt. Electrical energy is measured in kilowatt hours.

As an example of this calculation, if the voltage across a portion of an electrical circuit is 50 volts and the current flowing in the circuit is 600 amperes, the amount of power being consumed in the circuit is 30 kilowatts, or 30 000 watts. If the circuit is maintained for one hour, 30 kW-hr of energy will be expended.