Which statement best describes true power?

• A. The power that actually performs work in a circuit.
• B. The power stored in magnetic fields.
• C. The total power including reactive and true.
• D. The power dissipated as heat in a resistor.

Answer: (A)

Real power is the average rate at which energy is actually delivered to a circuit element and converted to useful work or heat. It represents what the circuit truly consumes. In AC systems, voltage and current can be out of phase because energy is alternately stored in magnetic and electric fields; that stored energy leads to reactive power that does not do net work over a cycle. The portion of the total power that does work is P = VI cosφ; when the circuit is purely resistive, φ is zero and all input power becomes real power. The apparent power, S = VI, combines real and reactive power and is larger than the real power unless the circuit is purely resistive. Thus, the statement that best describes true power is the one that defines it as the power that actually performs work in the circuit.