Real Power (P) in a circuit is calculated as V × I × cos(phi) and is measured in Watts. Which of the following expressions represents Real Power?

• A. V × I × sin(phi) in VARS
• B. V × I × cos(phi) in Watts
• C. V × I in Volt-Amperes
• D. I × R in Watts

Answer: (B)

Real power is the portion of electrical power that actually does work, and in AC circuits it’s found by projecting the instantaneous power onto the voltage direction. This gives P = V × I × cos(phi), where phi is the phase angle between voltage and current and cos(phi) is the power factor. The unit is watts, reflecting the actual energy transfer over time.

The part with sin(phi) corresponds to reactive power (measured in VARs), representing energy stored and returned by the circuit’s reactance. The product V × I without the cos(phi) is the apparent power (measured in VA), which combines both real and reactive components. The I × R expression isn’t the general real power formula; it doesn’t account for the actual voltage and phase relationship except in special cases, whereas real power in a circuit is P = VI cos(phi).