[wmf]

One thing to keep in mind is that one rack holds 90 servers and each battery rack feeds 180 servers, so (at least in Facebook's case) that's the basic unit. If this stuff takes off I would expect someone to produce a smaller 30-slot rack, though.

[ewams]

No. Each rack holds 30 servers and each triplet holds 90. The battery cabinet can support 2 triplets.

http://opencompute.org/datacenters/