Power Dissipation Calculator (P = I²R, V²/R, VI)
Power dissipated in a resistor or component from any two of voltage, current and resistance, with energy per hour and a resistor wattage-rating check.
How it works
P = V × I = I² × R = V² ÷ R
Electrical power turned into heat in a resistive component follows Joule’s law; the three forms are the same relation combined with Ohm’s law. The I²R form explains why transmission lines run at high voltage — halving current quarters the loss. Resistors should run at no more than half their rated wattage in still air; the rating is for 25 °C ambient and falls at higher temperatures.
Worked example
12 V and 0.5 A: P = 6 W, R = 24 Ω. A 10 W resistor would run at 60% — better to use 20 W or add a heatsink.
Frequently asked questions
Is all this power really heat?
In a resistor, yes. In a motor or LED some becomes work or light; the formula gives total power drawn.
Why does a 100 W bulb not blow a 1 A fuse at 230 V?
It draws 100/230 = 0.43 A. Fuses are rated by current, so convert with I = P/V.
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This calculator is for educational purposes. Double-check important results before acting on them.