Is there any downside to having way more PSU capacity than I need?

[Joe_Papa]

I'm running an ML350 G6 server. It has redundant 750W PSUs. According to the power meter readings from ILO2 it has only used 193W maximum in the last 24 hours.... I've seen it get as high as 226W, but nowhere near 750W.

The server currently is running the following hardware:

• (2) Xeon 5675 processors (12 cores, 24 threads)
• 72GB of ECC RAM
• (1) 1TB SSD
• (8) 300GB 2.5" SSDs in a RAID6 array (1.6TB of storage)

I may add another drive cage for 8 more drives in the future, but I think that's probably as much hardware as I'll ever add to this server.

I could replace the two 750W PSUs with two 460W PSUs if there was a reason to do so (more efficient, cooler, or something else beneficial). It would only cost around $30 so it's not a big deal, but I won't do it if there is no reason to. 

[parnassus · Accepted solution]

Hi! downsizing a PSU is not totally unreasonable these days...but, if done, it should be done only after knowing exactly what is the "efficiency curve" of involved PSUs (and the way they work together -> balancing?) and what will be the maximum power supply load they should be able to sustain during normal operation.

Let we suppose:

• 300W is the maximum power request (at the worst condition)
• highest efficiency (say nearly 90%, between 85% and 89%) reached working at about 50% of the declared PSU maximum power rating.
• one PSU involved

then a 600W PSU should be recommended.

Probably there are also other factors (engineering, reliability, etc.) to consider.

[Joe_Papa]

Thanks for this information. I appreciate it.