9.2.3—P = VI, P = I 2R and P = V 2 / R

For a component, power P=VI. Combining with V=IR gives P=I²R and P=V²/R, with the appropriate measured voltage and current.

Choose the form that uses known quantities and distinguish input power from useful output. These equations assume the component’s voltage-current relation at that operating point.

A 6 Ω resistor carrying 2 A dissipates P=I²R=24 W, also equal to VI when V=12 V.

Do not use P=V²/R for a non-ohmic device with a fixed resistance assumption unless its operating-point resistance is known.