Power is how fast energy is converted — into computation, light, sound or, unavoidably, heat. In a DC circuit it is simply the voltage multiplied by the current:

P = V × I

Combined with Ohm's law, power in a resistance can also be written as P = I² × R or P = V² / R. Those forms explain why heat rises so quickly with current: double the current through a resistance and the heat goes up four times.

Power in a laptop

  • The adapter is rated by the power it can deliver: a 65 W adapter at 20 V supplies up to 3.25 A. The laptop's charger IC limits how much it draws so the adapter is not overloaded.
  • The battery stores energy, measured in watt-hours. Power drawn from it at any moment depends on what the laptop is doing.
  • The CPU and GPU are the biggest consumers. Their power is set by voltage and current on the core rails, and it changes thousands of times a second.
  • Regulators convert power from one voltage to another. A buck converter taking 20 W at 19 V delivers a little less than 20 W at 1 V — at more than 19 A.

That last point surprises many beginners: lowering the voltage raises the current for the same power, which is why core rails need large inductors, several MOSFETs and wide copper.

Power and heat

Every real component wastes some power as heat, known as power dissipation. A regulator that is 90% efficient and delivers 20 W wastes about 2 W, which its MOSFETs and inductor must shed. A shorted capacitor fed with 5 A through a small resistance can turn several watts into heat in a part the size of a grain of rice — enough to make it glow on a thermal camera or melt rosin on its surface.

Reading power during diagnosis

A USB-C power meter or a bench supply shows the board's total power draw. Typical patterns:

  • A fraction of a watt while off but plugged in — standby rails only.
  • Steps upward when the board powers on, then fluctuating values while booting.
  • Several watts at the input with a dark screen — the board is running but something such as the display path is not working.
  • A sudden jump to the supply's limit — likely a short somewhere.

The numbers are only rough guides; compare with a known-good unit of the same model when you can.

Units and prefixes

  • milliwatt (mW) — one thousandth of a watt, typical for small chips in standby;
  • watt (W) — the scale of adapters, CPUs and backlights;
  • watt-hour (Wh) — energy, not power: one watt for one hour.

Common mistakes

  • Confusing power (W) with energy (Wh). An adapter is rated in watts; a battery in watt-hours.
  • Using a lower-power adapter than specified. The laptop may run but refuse to charge, or throttle heavily.
  • Ignoring heat when replacing a part with a smaller package: it may carry the current but not shed the power.

See also efficiency for how much power regulators lose along the way.