Electrical Exam · Free practical tool

AC power calculator

Separate real power from apparent and reactive power for a sinusoidal load. For three-phase questions, identify line voltage and current explicitly.

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The method

Apparent S = VI single-phase or √3 VLIL three-phase; P = S × PF; Q = S√(1−PF²).

Before using the result

  • Sinusoidal steady state with a lagging load. Balanced three-phase inputs are RMS line voltage and line current; harmonic distortion is not modelled.

Worked examples

These original training examples illustrate the method for Master Electrician Advanced Knowledge Review (ME). Use the matching “Load example” button above to explore a variation; example values are not equipment settings or prescribed criteria.

1. Worked example

Separate real power from apparent and reactive power for a sinusoidal load. For three-phase questions, identify line voltage and current explicitly.

Supply
Single phase
Voltage (V)
230
Current (A)
10
Power factor (lagging)
90 %
  1. S = 2300 VA.
  2. P = 2300 × 0.9 = 2070 W.
  3. Q = 2300 × √(1 − 0.9²) = 1002.5467570143547 var.
  • Real power: 2,070 W
  • Apparent power: 2,300 VA
  • Reactive power: 1,002.547 var

2. Comparison example

Compare a different supplied condition using the same method. For Electrical Exam, the important check is that the input meaning, units and method assumptions still match the problem.

Supply
Balanced three phase
Voltage (V)
400
Current (A)
10
Power factor (lagging)
90 %
  1. S = 6928.203230275509 VA.
  2. P = 6928.203230275509 × 0.9 = 6235.382907247958 W.
  3. Q = 6928.203230275509 × √(1 − 0.9²) = 3019.933774108299 var.
  • Real power: 6,235.383 W
  • Apparent power: 6,928.203 VA
  • Reactive power: 3,019.934 var

Check your understanding

1. Which relationship is used?

Apparent S = VI single-phase or √3 VLIL three-phase; P = S × PF; Q = S√(1−PF²).

2. Which assumption should you check first?

Sinusoidal steady state with a lagging load. Balanced three-phase inputs are RMS line voltage and line current; harmonic distortion is not modelled.

3. How should you verify a different unit system?

Convert every input to the displayed calculation units first. Equivalent physical inputs must give equivalent results; a unit label alone does not convert a number.

Continue with the matching exam topic

This tool connects to Master Electrician Advanced Knowledge Review (ME) free practice. Work through a question independently, then use the calculation steps to identify where your method differs. The tool stays free whether or not you choose paid preparation.

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