My Next Datacenter: No DC Power For Me

When the green data center movement started a few years ago, I started to see claims that switching our data centers to DC power would save us 20 or even 30 percent on our power costs. I'm not buying it. Not only do I think the 20-30 percent savings claims are, shall we say, somewhat exaggerated, but I don't think there's 20 percent power waste in the power conditioning and distribution systems in most data centers. As we continue the My Next Data Center, series I'll explain why there's "No DC For Me."

A conventional AC power system takes line power, typically at 480 or 208V, rectifies it to DC to charge the batteries and then inverts back to AC to supply the data center gear. The server power supply then rectifies it back to DC. The argument for DC distribution in the data center is that eliminating the last two stages of DC to AC and back to DC will save lots of juice. Power can then be distributed around the data center as 48VDC, just as it has been for decades in telephone company central offices and some corporate PBX rooms.

The problem with that argument is that today's AC gear is pretty darned efficient. Eaton's BladeUPS is over 95 percent efficient at any load over 20 percent of its capacity, and since it's modular, there's no good reason to run at less than 20 percent capacity. Server power supplies have also gotten more efficient. The SuperMicro 2u servers we run in the lab have power supplies that are 80 Plus Gold certified which makes them 88-92 percent efficient depending on load. They also have a power factor of .9 which further reduces the load on the UPS.

Then there's the fat cable problem.  Ohm's law tells us that the voltage drop across a length of wire is proportional to the current and the wire's resistance but not the input voltage. Sending 20amps of current over a 50 foot length of 12 gauge wire will result in a voltage drop of 1.6V.  That's about 0.7 percent loss for 208V power, but a 3.4 percent for 48V. To get the voltage loss down to a reasonable 0.8 percent, we'd have to upgrade to six gauge wire that's twice as big and more expensive to install because it's so stiff.

The solution used at Syracuse University's new demonstration data center is to send power to each rack at 575VDC. Higher voltages mean thinner cables, but almost all equipment that will run on DC at all is designed for 48V so they convert it to 48V in the rack.  The converter is 95 percent efficient but then so is a modern AC UPS.Also remember that while data center gear does run exclusively on DC internally (unless you still have 8" floppy drives that use AC motors) they don't run on 48V. Disk drives use five and 12VDC, while most of the semiconductors are down under two volts. That means servers will still need power supplies to convert 48V to the internal voltages they need, and those power supplies are never 100 percent efficient. When Dot Hill sent out a press release a year or so ago announcing their DC-powered disk arrays, I spoke to one of their engineers who told me the AC power supplies were 90 percent efficient and the DC supplies 92 percent.

The final argument against DC is that there is just a lot less equipment available with DC power supplies. Yes, I can buy servers and storage from the major vendors, but not every model, and I don't want to have to limit my choices on KVM consoles or anti-spam appliances to those with DC power available. Even if I build a DC data center, I'd have to swap out the power supplies in all my existing gear to move it in and supply AC to some racks to hold the equipment I couldn't buy for DC.

With a top-of-the-line AC system being 85 percent efficient from the utility pole to the server motherboard, and DC systems being maybe five percent better, it's just not worth it. Of course I expect e-mails and comments from the DC fans trying to prove me wrong as soon as this is posted.