ElectroHub

Electrical Power Calculator

Three forms of the same relationship, picked automatically from whichever two values you supply. Power is what sets component ratings, heatsink size and your electricity bill.

Power — P = V × I = V²/R = I²R

Enter any two of voltage, current or resistance. Power and the missing value are computed.

Enter at least two values.

Common trap: A resistor's wattage rating (¼ W, ½ W…) is the most it can survive dissipating, not what it consumes. Pick a rating at least 2× the computed power or it will run hot and drift — or burn.

The formula

P = V × I = I² × R = V² / R

P
power (watts)
V
voltage (volts)
I
current (amperes)
R
resistance (ohms)

Worked example

12 V across a load drawing 500 mA.

  1. P = V × I = 12 × 0.5 = 6 W
  2. The load looks like R = V / I = 12 / 0.5 = 24 Ω
  3. Check with the other form: P = V² / R = 144 / 24 = 6 W ✓

6 W, from a load equivalent to 24 Ω.

Where you'll use it

Choosing a resistor wattage, sizing a supply, or deciding whether a part needs a heatsink. The I²R form is the one that matters for cables and windings, where resistance is fixed and current varies.

The laws behind it

Parts this applies to

Resistor (fixed)Capacitor (aluminium electrolytic)Capacitor (tantalum)LED (light-emitting diode)MOSFETRectifier DiodeSchottky DiodeZener Diode

Related calculators

Ohm's Law CalculatorV = I × RJoule Heating CalculatorH = I² × R × tEnergy and Electricity Bill CalculatorUnits (kWh) = (Watts / 1000) × hoursCapacitor and Inductor Stored Energy CalculatorE = ½ × C × V² (capacitor)