About PC power calculations

A LETTER WE PRINTED yesterday compared today's high-powered CPUs to the Detroit Muscle Cars of the '60s and early '70s. Though the writer made valid points, his math was a little off, no disrespect intended.

The letter posited 4 PCs consuming 300 watts each running 24x7. Simply multiplying these numbers came up with 4x300x24x7 = 201,600 watts -- or 201.6 Kw. This is correct... for one week, but our correspondent called this an annual figure. He did the same math for 4 notebook PCs burning 75 watts each running 24x7, which yielded 50,400 watts -- or 50.4 Kw -- again for a week, not annually. The relative power consumptions are of course right, given the assumptions, but the scaling is off. This tends to weaken his economic point, if one reads his numbers uncritically.

In particular, he asks whether the difference in power consumption will plausably outweigh the price differential between an inexpensive desktop PC and an expensive notebook PC, over the life of the two machines.

So let's do the math right based on our writer's assumptions and see.

Desktop PCs:
4 x 300w x 24hr x 365days = 10,512 Kwh/yr.

Notebook PCs:
4 x 75w x 24hr x 365days = 2,628 Kwh/yr.

We need two more assumptions: PC lifespan and average cost per kilowatt hour (Kwh -- power utilities charge for kilowatt hours). We will assume the PCs last two years and power costs 5 cents per Kwh. Fair enough?

Desktop PCs:
10,512 Kwh/yr x 2 yrs = 21,024 Kwh.
21,024 Kwh x $0.05 per Kwh = $1,051.20.

Notebook PCs:
2,628 Kwh/yr x 2 years = 5,256 Kwh.
5,256 Kwh x $0.05 per Kwh = $262.80.

Power cost savings for Notebook PCs:

$1,051.20 - $262.80 = $788.40 for two years.

So our writer's hypothetical family might save enough on electric bills over two years to find a good used notebook PC on Ebay for their dog. It would not be nearly enough power cost savings to make up the difference in price between four desktop PCs and four notebook PCs, bought new.

However, power cost savings increase at higher rates and people keep PCs longer than two years, lately. So the economic balancing of a desktop vs a notebook PC will differ in other circumstances. If one suffers with an electric utility that charges 15 or 20 cents per kilowatt hour and maybe springs for a new PC every three or more years, perhaps the calculations might argue for an expensive notebook over a power-hungry desktop PC.

The economics are different in a business setting, as our correspondent rightly observed. Multiplying the above costs by 100 PCs yields $105,120 for desktops, $26,280 for notebooks. No wonder businesses remind their employees to turn off all PCs and lights before going home at night.

Of course, most people do turn their PCs off at night, and notebook PCs suspend automatically when left unused (or at least, good ones do).

But our letter writer's closing argument is nonetheless attractive: the electricity cost savings using a 15 watt Transmeta CPU can be compelling compared to the cost of a PC Muscle Car, either desktop or notebook. As our correspondent hinted, to assess your PC power costs, do the math.

The INQUIRER's earlier article publishing this letter is here. µ