For a very long time it is possible to buy almost anything data center-worthy in version with 48V DC power supply (although often with significant premium) and

For a very long time it is possible to buy almost anything data center-worthy in version with 48V DC power supply (although often with significant premium) and most collocation facilities provide 48V DC power. This has nothing to do with efficiency but with 48V being the standard voltage in telco environments, because it's 4 car batteries in series. Phones need DC and until about 50's it wasn't economical to rectify mains AC with enough power to run phone switch and without harmful noise and AC ripple, so phone switches were generally battery powered (today it's typicaly generated by rectifing mains with battery backup). Servers and networking equipment around these switches generally shares these same power circuits. Problem with 48V is that for any significant powers the wires get very thick very quickly. Many larger server installations noticed that (1) normal AC server SMPS will happily run from 150-300V DC without any modification (reliability is an issue, omitting input rectifier and PFC is good idea) (2) there is almost nothing in today's servers that need any other voltage than 12V. Thus there are various approaches to DC power (for increased efficiency, not telco compatibility) in datacenters: * DC power supplies with DC voltages around normal mains voltage (IIRC Facebook) * power supplies with inputs for two different voltages (AC mains + sub-100V DC for backup power from batteries) * battery backup inside the server on 12V power bus (Google) * shared 12V DC power supply for multiple servers (almost any blade solution, although I've seen that done with normal 1U servers) * also I vaguely remember some proposal to replace local 12V->something buck converters in modern server with larger input voltage.