Efficiency, Losses and the Heat You Have to Remove
Efficiency is quoted as a single percentage, which makes small differences look unimportant. The number that matters to your enclosure is not the efficiency but the loss — and the two behave quite differently.
Turning efficiency into watts
A supply delivering 100 W at 90 per cent efficiency draws about 111 W and dissipates roughly 11 W as heat. The same output at 94 per cent dissipates about 6 W. The efficiency improved by four points; the heat you have to remove fell by nearly half. In a sealed cabinet holding several supplies, that is the difference between passive cooling and needing a fan.
The quoted figure is a best case
- It is normally stated at full load and nominal input. Efficiency usually falls at light load, so a supply running at a quarter of its rating may be well below the headline.
- Low input line costs efficiency, because currents are higher for the same power.
- Standby losses are separate. Equipment that idles most of the time is judged on no-load consumption, not full-load efficiency.
Sizing the enclosure
Add up dissipation for every source in the cabinet at your actual operating points, not the ratings. Remember the feedback loop: losses raise the internal temperature, higher temperature reduces efficiency and forces derating, and shorter capacitor life follows. Oversizing a supply so it runs at half load is a common and effective tactic — but check the efficiency curve, because some units are noticeably worse there.