How to Calculate Electric Power (P = VI, I²R, V²/R)

Calculate electric power with P = VI, P = I²R, and P = V²/R. Pick the right form, fix mA/kΩ traps, and use a free electric power calculator.

What Electric Power Asks

Electric power is how fast electrical energy is transferred in a circuit. A bulb rated 60 W converts 60 joules of electrical energy into light and heat every second.

The basic formula is P = V I — voltage times current. When the load obeys Ohm’s law, you can also write P = I²R or P = V²/R. All three give power in watts when V is in volts, I in amperes, and R in ohms.

If you already have two of V, I, R, or P, open the free electric power calculator first, then return here for which form to pick and unit traps.

The Formulas

  • P = V I — power from voltage and current.
  • P = I² R — power from current and resistance.
  • P = V² / R — power from voltage and resistance.
  • I = P / V — rearrange when power and voltage are given.

    • These resistor forms come from substituting Ohm’s law V = IR into P = VI. Use the form that matches the two values you already know — do not mix forms carelessly.

    Electric Power vs Mechanical Power

    Both are measured in watts, but the inputs differ. Mechanical power is often P = W/t or P = Fv (work or force and speed).

    Electric power uses circuit quantities. If a lifting problem asks for watts, use mechanical power. If a resistor or bulb asks for watts, use P = VI. See also the mechanical power guide and the

    work vs power exam traps.

    Link to Ohm’s Law

    Need V, I, or R first? Use the Ohm's law calculator with V = IR, then plug into the matching power form.

    For two-resistor networks, find equivalent resistance with the series-parallel calculator before computing power for the combined load.

    Units

  • V → volts (V).
  • I → amperes (A). Convert mA → A (÷ 1000).
  • R → ohms (Ω). Convert kΩ → Ω (× 1000).
  • P → watts (W). 1 W = 1 J/s.
  • Leaving mA or kΩ unconverted makes P wrong by large factors.

    • Worked Examples

    Example 1: P = VI

    V = 12 V, I = 2.0 A. P = (12)(2.0) = 24 W.

    Example 2: P = I²R

    I = 0.50 A through R = 40 Ω. P = (0.50)²(40) = 0.25 × 40 = 10 W.

    Example 3: P = V²/R

    V = 120 V across R = 240 Ω. P = (120)² / 240 = 14400 / 240 = 60 W.

    Example 4: find current from power

    P = 60 W, V = 120 V. I = P/V = 60/120 = 0.50 A.

    Common Mistakes

  • Mixing milliamps with ohms without converting to amperes.
  • Using P = I²R when you only know V and R (use P = V²/R instead).
  • Confusing mechanical P = W/t with electric P = VI.
  • Forgetting that power in a resistor becomes heat (I²R losses).

    • Check It on the Calculator

    Electric power calculator — P = VI, I²R, V²/R.

    Ohm's law calculator.

    Series-parallel resistance calculator.

    Mechanical power calculator — P = W/t.

    Learn P = VI and confirm every homework number on the calculator.

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