“40% more efficient? really? this is what the numbers say, but it is not what I have seen in testing. If you use reasonably efficient power supplies (80+ gold) blade servers often use as much power as measured at the PDU as rackmount servers do , and they are much cheaper, and much more flexible. (Note, you do get a boost in efficiency using 208v power instead of 120v power, but nearly all hardware built in the last 10 years can handle both.)”

which is the opposite of what I had to say; while blade servers and rack-mount servers consume a similar amount of power, it is rack mount servers that are much cheaper and more flexible, not blade servers.

Now, if you want to save power, start looking at CPUs with better performance per watt values. Consolidate your old servers; Use virtualization if consolidating on to the same server is too complex.

Most blade enclosures have at most 16 blades. Sometimes you have to reduce that to 8 if you want disk. It’s trivial to fit more than 16 servers in a rack (assuming you have the power budget)

Also, blades nearly always require external disk for most applications (most blade servers I have seen max out with 2×2.5″ SAS drives) rackmount servers give you the flexibility to use cheap local storage.

The “blades are too dense” argument is not sensible. Put less blade chassis per rack if you cannot handle the density. Even at one chassis per rack (say 4-5kW), you are still deploying a very significant compute resource, and most likely getting more compute than you would otherwise deploy in the entire rack.