Computer power supplies

jils (who has an excellent software development blog at dev.notoptimal.net) notes an efficient power supply can make a big difference. By the chiming in with this sort of thing is greatly appreciated. The blog format is not ideal it was just easy way to put stuff the web, but I really hope that people chime in ways of measuring and reducing energy usage, and for evaluating environmental decisions.

One other item of note on quantitating computer power usage, the power supply rating (say 350Watts) is the output number, which means you have to factor in the efficiency rating to determine how much power it will conceivably draw. While most computers don't draw the peak amount, you may be wanting to know this for peak loading if you are sizing a UPS or solar system.

So a 80% efficient 350W power supply can draw 350/.8 = 437.5 watts and a 60% efficient power supply can draw 583 watts. That one component can make a big difference especially if you leave your computer on all the time. Keep in mind that the .2 or .4 "inefficient" part of the power supply all goes to heat which means you have to run your a/c more.

Computer power supply efficiencies are actually variable based on load. So really in the end to know how much power you're drawing you have to measure it.

So like I always say, there's lots of things you can do to reduce your energy usage, but how do we evaluate the choices with our own constrained resources (read: wallet)

Is it better to spend an extra $100 to get a 80% efficient supply or is better to spend an extra $100 to get a lower power processor? The lower power CPUs out today are design to be in a 65 watt (thermal design envelope) meaning that they aren't meant to put out more than that. Generally that applies to a range of processors so the fastest ones will put out 65 watts (which is the same as the amount of power they draw) and the slower lower power ones might be in the high 50s. The regular power processors range from 80 as high as 120 watts (though those are typically workstation or extreme processors which aren't in the same purchase category). So lets say you save 30 watts by going with the low power cpu. For a 350 w power supply the difference between 70% efficient (about average) and 80% efficient is 125 watts.

Score one for the power supply.

AMD Cool n quiet

A friend of mine (jils) built a home theater pc with the goals of it being quiet and cost effective. Quiet computers are achieved by eliminating fans, which implies having low heat, which means low power.

With AMD CPUs there is a software program called cool n quiet, that will reduce the clock speed of the processor when it's not being used. On intel computers this is generally known as speedstep and doesn't usually require software.

He describes his home theater pc designed for watching HDTV on his 720P LCD rear projection tv like this:

based on an A64 3500+ Winchester CPU So it is not one of the super low-power jobs, but still much cooler than a P4 of that generation. http://en.wikipedia.org/wiki/Athlon_64#Winchester_.2890_nm_SOI.29

The Mobo chipset is an Nvidia 6150 with integrated Geforce graphics sharing the onboard RAM.
80 Gig Seagate Barracuda 7200.7 IDE HDD
300 Watt Antec PS
Avermedia analog Tuner Card
Dvico Fusion HD Tuner Card

He reports the following power usage without the software:
All Measurements in Watts. AMD Cool and Quiet NOT Enabled
S3 Standby: 6
Idle: 81-82
High Load: 98 W (720p x.264 Playback inside of the MCE Interface).



With Cool & Quiet software running, the 3500+ clocks down to 1000Mhz and up from there depending on load.

IDLE: 69 Watts
Watching 640px XviD: 69 Watts (no extra CPU needed for viewing).
Load: 720p x.264 MKV: 90-96 Watts


So he saves 12 watts on idle, and a few watts when it's decoding hdtv. Also interesting to note that the system is effectively idling when it's playing a dvd, because the cpu is still fast enough at the lower clock speed so it doesn't need to increase the speed (and thus power)


The intel core 2 duo chips are very cood at power reduction (probably better) but they can be hurt by the chipset taking up a lot of energy.

The AMD chip jils chose was very cheap, and thus also there's implied environmental savings as it probably took less silicon to produce.

Note that letting his computer go to standby (from which windows media center can wake up to record shows) saves a tremendous amount of power and still provides instant on.

He mentions S3 state which is a lower power mode of standby.

Note that screensavers don't save energy! only having your computer turn off your monitor, hard disks or go to standby (best) does! hibernate is even better.

However if you have a AMD computer look in to cool n quiet to reduce your power usage while you are using your computer!