I’ve just read an interesting post on TomsHardware.com about a solar powered PC . It describes all the steps involved in creating a modern high-performance low-power computer.
They have a lot of interesting information. One surprising fact (from page 3) is that the PSUs tested (both for AC and DC input) were more efficient when idle (I expected the greatest efficiency to be when under load).
An AMD processor was chosen due in large part to the fact that chipsets in suitable motherboards used less power. For the CPU itself Intel had a competitive offering but no matching motherboard was power efficient enough (from page 7).
Page 8 documents how using a cooling fan (instead of passive cooling) reduced the power requirements of the CPU to such a degree that it always saved power use overall. Why do CPUs take less power when they are cooler?
Page 9 mentions that a small passively cooled video card can draw 88.5W when idle! That sucks pretty badly, it seems that having a video controller integrated with the motherboard is the way to go if you want to save power.
It’s interesting to note how much energy can be used by RAM. Page 13 shows that the difference between 2*1G and 2*512M can be as much as 3.4W and that the difference between different brands of RAM for the 2*1G can make as much as 1.2W difference. Their final system drew 61W when idle, my latest 64bit system takes 52W when idle  (which compares to the 38W of their system without a monitor), so we are talking about 9% of system power being saved by using less RAM or 3% being saved by using a different brand of RAM.
The summary of hard drive power use on page 14 is interesting, the fact that 2.5 inch laptop disks use less power than 3.5 inch desktop disks is hardly surprising, but the difference when idle is very surprising (apparently one of the 3.5 inch disks spends 8W on turbulence and friction in the bearings). It’s unfortunate that they didn’t compare any of the server-class 2.5 inch disks, it was about 6 months before the article was written that HP announced that in future they would cease shipping 3.5 inch disks and only use 2.5 inch disks (I wonder if this is related to all HP’s recent work on server cooling). Rumor has it that many server class 3.5 inch disks have platters that would fit into a 2.5 inch case because at high rotational speeds a larger diameter platter would not be strong enough.
The information on DVD power use on page 15 is quite shocking. From now on when I install machines as servers which don’t have a need for for a CD-ROM drive I’ll remove the drive prior to deployment. Even if it saves only 0.47W then it’s still worth doing on a machine which uses less than 40W! An additional benefit of this is that it might speed up the boot process as the system won’t need to check for a bootable CD.
It’s unfortunate that most computer parts don’t have published documentation on how much power they draw. Even if you don’t want to run on solar power there are still significant benefits to saving electricity (including reducing the noise from cooling fans and heat problems in summer). If technical data was published then people could make informed decisions about which parts to buy.
Update: Changed the percentage savings for different types of RAM to be based on the system power use without the monitor. I’m most interested in saving power for servers and for idle desktops (running a desktop machine 24*7 is pretty common) so most of the time the monitor will be turned off.
It’s interesting to note that they power their system uses is about the same as a P3 system and could be less if they used a different hard drive.