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Electrical Units – Power, Voltage, Current, and Energy

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.

The same triangle approach applies here:

      U
    -----
    R | I
  • U = R x I (Voltage = Resistance x Current)
  • R = U / I (Resistance = Voltage / Current)
  • I = U / R (Current = Voltage / Resistance)

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

Electrical Work (Energy)​

Electrical work W (often called energy, sometimes written as E) is the amount of energy a device consumes or produces over time. The SI unit is the Joule (J), but in everyday electrical use it is more commonly given in Watt-hours (Wh) or Kilowatt-hours (kWh).

The basic formula is:

  • W = P x t (Work = Power x Time)

Replacing P with U x I gives a form that calculates W directly from U, I, and t, without needing to calculate P first:

  • W = U x I x t

Common Units of Energy​

UnitSymbolEquals
JouleJ1 W x 1 s
Watt-hourWh3,600 J
Kilowatt-hourkWh1,000 Wh = 3,600,000 J
info

The symbol W is used for both the unit "Watt" (a unit of power) and the quantity "work". Context usually makes the meaning clear. Some sources use E for "work" to avoid confusion.

Examples​

Example 1: A 60 W light bulb runs for 5 hours. How much energy does it consume?

W = P x t = 60 W x 5 h = 300 Wh = 0.3 kWh

Example 2: A device draws 2 A at 230 V for 30 minutes. How much energy does it use?

W = U x I x t = 230 V x 2 A x 0.5 h = 230 Wh

Example 3: An electricity bill shows 250 kWh consumed at €0.30 per kWh. What is the total cost?

Cost = 250 kWh x €0.30/kWh = €75.00