Muscle Cars and CPUs: heated debate continues

WE'VE HAD A LOT of correspondence about this letter: Are CPUs like Muscle Cars.

Here's what Dan at Dan's Data has to say, today:

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The article [by Egan Orion} here largely corrects [the original letter here] although paragraph two needs a couple of "watts" and "KW"s changed into "watt-hours" and "KWh") - but one important further correction remains.

A computer with a 300 watt PSU almost certainly does not draw 300 watts of power, any more than a car with a 300 horsepower engine is always running with its throttle wide open.

Only if your 300 watt PSU is almost fully loaded - in which case the computer will probably misbehave quite atrociously, as PSU maximum ratings are generally quite optimistic - will it _deliver_ its full rated power; to deliver 300 watts to PC components (assuming that it can), it actually has to draw more than 300 watts from the wall socket, since it's not 100% efficient at converting AC into multi-voltage DC. But it's not likely to actually be drawing anything like that much power, except perhaps for a moment at startup.

A more realistic power consumption figure for a _stacked_ PC, working hard, is 150 to 200 watts, plus another 75 to 150 watts for a CRT monitor, depending on its size and vintage. Very few PCs work hard all day, and few PCs qualify as stacked machines (some older machines with power-hungry components can draw just as much as a new top-flight desktop box, but most office PCs don't get close). A realistic power consumption figure for a single office PC with CRT monitor, when it's being used, is around 150 watts. Or a lot less; the 400MHz Celeron box with 15 inch monitor that power from a jerry-rigged UPS at this page only draws about 100 watts from the DC side of its inverter, during startup.

That computer's got the same specs as a lot of current business machines. But let's say 150 watts, for the sake of argument.

Allow for ACPI power saving, and the fact that NT-series Windows flavours instantly put the CPU in power-save mode whenever it doesn't need to do anything, and I don't think it's unreasonable to peg the average power consumption of a 150-watt-peak office box, even if it's turned on all day and used for a whole eight hours each day, and does not have its monitor manually turned off, at about 1.6KWh per day. For 16 hours of the day, it'll only be drawing about 25 watts, if that.

The calculation in this article is less unrealistic than it might be, since it compares PCs to laptops and thus implicitly includes monitors in the PC calculation - and a CRT monitor can easily draw more power than the computer it's connected to - but it still over-estimates.

Given the above more accurate assessment of a PC's power consumption, the four-PCs-for-a-year figure drops from 10,512kWh to only 2,336kWh.

The quoted 75 watt figure for a laptop is similarly exaggerated, mind you; it's not unreasonable to still say that a laptop will have about a quarter of the draw of a PC (with a CRT monitor on the PC, and with the laptop using its LCD), so the four-laptops-for-a-year figure would be down around 584KWh. In any case, though, the smallness of the numbers means the laptops will only save you $87.60 a year, given the 0.05c/KWh power price quoted. The total power expense for the four PCs will be $116.80; that's the most you could _possibly_ save per year, and what you'll _actually_ save per year is, given the above calculations, only 22% of what's estimated, based on 300 watts per PC and 75 watts per laptop.

Clearly, therefore, switching your PC CPUs from relatively power-hungry Intel or AMD chips to Transmeta (or, more plausibly, Via C3) chips will not save very much money, because not very much is actually being spent in the first place. The purchase price of the equipment, and ongoing service costs, are much more important; in both of these departments, laptops are seriously inferior to desktop PCs, though a company may well save money on service-staff back pain treatment by opting for portables rather than desktops with CRTs.

I talk more about all this in my column here.

Daniel Rutter
http://www.dansdata.com/ - hardware reviews and more!