What Is the Power Formula (P = V × I), and How Do You Calculate It?

Electrical power is the energy a component dissipates per unit time, and it's calculated with three different formulas. This article focuses on the calculation; for the concept of "what is power," see What Is Electrical Power? (Watts).

The Three Basic Power Formulas

All three give the same result; you pick the one that fits the values you have. They're derived from Ohm's law (V = I × R):

Worked Example 1: Device Power (P = V × I)

If an adapter supplies 9 V while the device draws 0.3 A: P = 9 × 0.3 = 2.7 W. The device uses 2.7 watt-hours of energy per hour (energy and consumption).

Worked Example 2: Resistor Heating (P = I² × R)

If 0.2 A flows through a resistor and its value is 47 Ω: P = 0.2² × 47 = 0.04 × 47 = 1.88 W. This power appears as heat; that's why at least a 2 W resistor should be chosen.

Worked Example 3: When the Load Voltage Is Known (P = V² / R)

A 100 Ω heating element with 12 V across it dissipates: P = 12² / 100 = 144 / 100 = 1.44 W.

Common Mistakes

• Choosing the wrong formula: in P = I² × R use the current, not the voltage; mixing them up throws the result off completely.
• Skipping a unit: putting mA or kΩ into the formula without converting to A or Ω gives a wrong power.
• Forgetting to square: in I²R and V²/R the value is squared; skipping this step gives a wrong result.