Recently I moved my computer components from a large full tower case to a smaller one that fits my new desk. In the process I decided to buy a new power efficient PSU to try to save a little bit on lost heat and electricity. My previous PSU was the Thermaltale TR2-430 (65+ efficiency) (specs) and I changed it for an Antec EA-430D Green (80 PLUS Bronze, so 82+ efficiency) (specs).

The old case also had 1 PATA cdrw and one PATA dvdrw and 5 80mm fans. In the new setup I replaced the drives with a single SATA dvdrw and the new case is running only 2 80mm fans at slower speeds. In summary, the new setup has one optical drive and 3 fans less than the old configuration.

I have a power meter (same as a Kill-a-watt) and these changes had the effect of increasing my idle power consumption from 89-91 watts to 99-101 watts, basically a 10 watts increase.

My question is what can explain that? And does anyone already had the same results with Antec EarthWatt PSU?


A computer power supply is an example of a switched-mode power supply. Most high-efficiency power supplies reach their peak efficiency at about 50% load. The unit has higher switching losses under low loads, and begins to saturate near full load, so the efficiency typically drops at these two extremes (see this link, click the "Tech Specs" for an example). Furthermore, the efficiency is typically a function of the input supply voltage.

As you can see from that curve, the unit runs less efficiently at lower loads. This is true of most power supplies, even newer ones (which have a greater efficiency near their rated loads). It's quite possible that your older power supply was more efficient at idle (i.e. 20-25% of the full load), and that your new one is more efficient under a load.

While your idle power consumption raised a small amount (10%), I would be willing to bet that your full load consumption dropped by a lot more.

  • I would have considered this but a 80+ Bronze PSU is stated to achieve 82% efficiency between 20% - 80% load and I'm really near these specs even at idle (20% load 86w). So at worst I was expecting 82% efficiency. On the other side the Thermaltake states 65% efficiency or more but because I'm near the 20% load I thought it was running near 65% efficiency. – jmbouffard Aug 8 '11 at 14:30
  • And also you mentioned the input voltage but I confirm that is was the same 120v electrical outlet during both measures. – jmbouffard Aug 8 '11 at 14:35
  • @jmbouffard can you provide power consumption results under a load? Also I just mentioned the voltage thing for completeness, didn't mean that it changed... – Breakthrough Aug 8 '11 at 14:38
  • I guess I should have checked the difference under load but I never expected to have these results so I never thought about recording consumption under load before to change the case and PSU. If I have some time in the next days I'll try to do it and post some more info... – jmbouffard Aug 8 '11 at 14:45

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