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Electrical Units – Power, Voltage, and Current

The Three Core Units​

SymbolQuantityUnit
PPowerWatt (W)
UVoltageVolt (V)
ICurrentAmpere (A)

The Power Triangle​

The three quantities are related by the formula P = U x I. Covering the quantity you want to calculate reveals the formula:

      P
    -----
    U | I
  • P = U x I (Power = Voltage x Current)
  • U = P / I (Voltage = Power / Current)
  • I = P / U (Current = Power / Voltage)

Ohm's Law​

Ohm's Law introduces resistance R (measured in Ohms, Ω) and connects it to voltage and current:

  • U = R x I
  • R = U / I
  • I = U / R

Combined Formulas​

Combining the power formula with Ohm's Law gives additional ways to calculate power:

  • P = U² / R
  • P = I² x R

Examples​

Example 1: A device runs at 230 V and draws 2 A. What is its power consumption (P)?

P = U x I = 230 V x 2 A = 460 W

Example 2: A 100 W bulb operates at 230 V. How much current (I) does it draw?

I = P / U = 100 W / 230 V ≈ 0.43 A

Example 3: A resistor of 50 Ω carries 3 A. What voltage drops across it?

U = R x I = 50 Ω x 3 A = 150 V