Power Calculator

Real power (watts) is the actual power consumed by loads to perform useful work. Apparent power (VA) is the total power supplied to the circuit, including both real and reactive components. Reactive power (VAR) oscillates between source and load without performing work but is necessary for magnetic fields in motors and transformers. The relationship is: S² = P² + Q², where S is apparent power, P is real power, and Q is reactive power.

For three-phase power calculations, use P = √3 × VL × IL × cos(φ) × η, where VL is line voltage, IL is line current, cos(φ) is power factor, and η is efficiency. For motors, use nameplate data: apparent power S = √3 × V × I, then apply power factor and efficiency. Account for load variations - power factor decreases at light loads. Always verify with actual measurements for critical applications.

Power factor indicates how efficiently electrical power is used. Poor power factor (below 0.85) increases current draw, requires larger conductors and transformers, causes voltage drops, and results in utility demand penalties. It affects conductor sizing per NEC requirements, transformer loading, and energy costs. Power factor correction using capacitors can improve efficiency, reduce infrastructure costs, and eliminate utility penalties while increasing system capacity.

For unbalanced conditions, calculate power for each phase separately using single-phase formulas, then sum the results. Use P₁ + P₂ + P₃ for total real power. Unbalanced loads create neutral current and reduce system efficiency.

Displacement power factor considers only the fundamental frequency phase relationship. True power factor includes harmonic distortion effects. For non-linear loads, true power factor is always lower than displacement power factor.

Harmonics increase RMS current without contributing to useful power, reducing power factor and increasing losses. Use true RMS measurements for accurate power calculations in systems with significant harmonic content (>5% THD).

Input power = Output power ÷ efficiency. For example, a 10HP motor at 90% efficiency draws 10HP ÷ 0.90 = 11.1HP (8.3kW) from the electrical system. Always use input power for electrical system sizing.

Apply NEC safety factors: 125% for continuous loads, 80% maximum loading for overcurrent devices. For motors, use 125% of nameplate full-load current per NEC 430.22. Consider ambient temperature derating per NEC 310.15(B)(2)(a).