power-supply efficiency

Power-supply efficiency is simply how much of the power you put in actually reaches the load, rather than being wasted as heat. It is the ratio of useful output power to total input power, efficiency = Pout / Pin, usually written as a percentage. An efficiency of 90 percent means nine watts in ten do the job and one is lost; 50 percent means you waste as much as you deliver. Every watt lost becomes heat that must be carried away, and (for battery gear) runtime thrown away.

For a linear regulator the answer falls straight out of the voltages, because the pass transistor passes the load current at the output voltage while dropping the rest: efficiency = Vout / Vin. So 5 V from 12 V is at best 5/12 = 42 percent, no matter how good the chip. A switching regulator breaks that link: it can be 85 to 95 percent efficient almost regardless of the voltage ratio, because it stores and transfers energy instead of burning the difference. Example: delivering 10 W at 5 V from 12 V, a linear regulator draws 24 W (wasting 14 W as heat), while a 90 percent buck draws about 11 W (wasting roughly 1 W).

Why it matters: efficiency drives heat, size, cost, and battery life — the four things a power design is judged on. It also sets the honest tradeoff at the heart of this whole field. A linear regulator is simple, cheap, and quiet but wastes (Vin - Vout) times current; a switcher is efficient but noisier and more complex. Pick linear when the drop is small and noise matters; pick switching when the drop or the current is large and efficiency matters; and watch the efficiency-versus-load curve, because most supplies are much less efficient at very light loads than at their rated point.