Earlier this year I replaced a PC acting as a file/print server in my "server room" (really just a closet under the stairs) with a Network-Attached Storage device. The problem was that even in its most idle state the PC would draw 60 watts, whereas the NAS I replaced it with draws 15 watts fully active and around half that when inactive (after as little as 10 minutes of no disk activity). Also in the server room is a printer, a DSL modem, and a 16-port router which distributes the internet connection from the modem to rest of the house. There is also a UPS that provides backup power to everything but the printer in the event of a power outage.
Upon replacing the PC with the NAS I realized that everything I had connected to the UPS ran off of 12 volts DC. Most of us know that conversions between AC and DC power are not very efficient (80% is typical). A UPS does what it does via two such conversions, one from the 120VAC from the wall outlet to around 12VDC to keep the batteries charged, and another from the 12VDC of the batteries to the 120VAC it provides to what's plugged into it (although a typical UPS does not use the second conversion unless it loses the incoming 120VAC, instead routing that straight to the output when present). Then each of the three devices plugged into the UPS uses an AC-to-DC conversion ("wall-wart") to convert the 120VAC available at the UPS outlets to the 12VDC it needs.
It struck me that there was too much conversion going on, and that a much simpler "DC UPS" could be devised, which would consume less power while working at least as well as what it replaced. I had an old 12-volt sealed lead-acid battery laying around and after some online searching, found a 13.5VDC high-efficiency power supply that could complete the system. (13.5VDC is around what you need to keep a 12-volt lead-acid battery fully charged, without risk of overcharging it.) I won't go into too much detail in this post, but my results were that the "typical" power consumption scenario (modem + router + NAS at idle) was reduced from 32 watts to 19 watts (about a 40% reduction), and the "NAS active" power consumption was reduced from 40 watts to 28 watts (a 30% reduction). This includes changing to a second DC power supply with even better efficiency, after learning a few things about the first one.
Granted, a 12-to-13-watt improvement is not that big of a deal, but I take the position that every little bit helps. And yes, the cost of a battery and power supply works against the cost-effectiveness of such an alternative, but remember that I took a UPS out of the picture, which also has a cost associated with it. Also, due to the lack of DC-to-AC-to-DC conversion between the battery and load, if a battery of the same capacity as that in the UPS it replaced were used, longer power outages would be tolerated with the DC UPS scheme (longer UPS "runtime"). I also run a few of my battery chargers off of this system, rather than use the wall warts they came with, increasing the energy savings however slightly.
I won't go into the gory details unless someone here expresses interest, but the bottom-line is: if you have some "ultimately" DC-powered devices (i.e. after the wall wart) that you need to provide constant backup power to, a DC UPS scheme such as I've described has the potential to reduce the overall power requirements of those devices by some 30%-40%. At $0.12/kWh, each watt of "always on" power consumption eliminated saves over a dollar a year in electricity costs. While not sounding like much, those watts can add up, and soon you are seeing the difference in your monthly power bills.
Edited by bobkart - 8/30/10 at 9:49pm