Geek Power

Computing power is getting so cheap. How cheap? So cheap that, for a home user, a dedicated firewall can be run on a Wifi Access Point like the Linksys WRT54G or a Netgear AP. What a lot of geeks used to do with a spare, full sized desktop PC can now be done on these “low power” devices. The cheap processing power allows someone to also save on electricity. This is good in many ways. The rest of this article comes from Keith Lofstrom. He explains the details.

We’ve recently discussed different alternatives for Linux based firewall routers. Wifi APs vs used laptops vs. old desktops vs. old Sun servers …

When making your choice, you should factor in a cost of around $20 per watt consumed. So a desktop PC consuming 40 watts is $400 more energy expensive than a laptop consuming 20 watts, which is $200 more expensive than a reprogrammed Wifi AP/router drawing 10 watts.

The calculation:

The incremental cost (the cost of consuming one more unit, not the average cost) of electricity on my power bill here in the hydropowered Northwest is around 10 cents per kilowatt hour. There are 8766 hours in a year, so the cost of a thousand watts for one year is $876.60, and the cost of one watt per one year is $0.876 .

Now for the addons. That watt heats your house or office, and it costs energy to get rid of it in the summer. The heat may be welcome in the winter, but natural gas is a cheaper way to generate a watt of heat, and better insulation is cheaper still in the long run. So the wintertime benefits are small, and the summertime costs are large. Let’s round the cost up to $1/watt/year to cover cooling, or the discomfort of the extra summer heat.

Then there is the social cost. Folks that don’t care about dammed rivers, power plant pollution, etc can skip this part, but most of us would rather minimize wasteful energy consumption because it makes the world more livable. There are those who would double or quadruple the cost of energy for these reasons, but I will go with a modest 50% adder. That takes us up to $1.50/watt/year.

When I consider a long term habit (like powering my internet router, or whatever replaces it as technology advances), I make cost choices based on many years of consumption. The choice of timespan is (again) a personal choice. Most economists suggest something called a “future discount rate”, that is, a unit of consumption now is X percent more important than a unit of consumption a year from now, approximately 2X percent more important than two years from now, etc. One reasonable assumption for X is the prime rate, currently 8.25 percent per year. Doing the math for an “exponential time series”, the “present cost” of a long sequence of $1.50 per year expenditures is $1.50/0.0825 or around $18/watt. The present cost is probably more, when you take into account the rising cost of electricity and other kinds of inflation, so let’s round that to $20/watt.

Time series also figure into recurrent capital expenditure, so the estimates get more complicated. Laptops are less reliable than desktops, so they will have to be replaced more often, increasing the cost of using them. On the other hand, low power laptops will last longer than higher power ones (heat kills), so power savings can lower your replacement costs. The same goes for desktops; for example, metal migration in processor sockets is an inexorable process (they are typically spec’ed for 5 years operation), so they will wear out in time. If you UNDERclock a desktop being used as a router, it will last longer. Dial the speed down to the minimum the BIOS permits, and you will save power and increase lifetime. Keep things clean to maximize efficiency and reduce overheating and fan failures.

If you use a Wifi AP, try to replace the very inefficent “wall wart” style power supplies with voltage-equivalent “switcher” power supplies. The switchers are much more efficient, and more tolerant of power line brownouts. You might be able to save 3 or 4 watts, which translates to $60 to $80. Laptop AC adapters are usually switchers, and adapters producing the common 5V or 12V power inputs for an AP can be found used on Ebay and elsewhere. You might have to change the plug. Note that APs and routers can often tolerate a fairly wide range of voltages; most 12V Linksys gear can take up to 20V with no problems, and up to 35V with some internal modification. These 12V units power up nicely off the 17V adapters used by many IBM laptops, and will operate down to 8V if supplied by a well-regulated switcher.

Yes, the above analysis is oversimplified, your mileage may vary. But the bottom line is that energy costs are an important part of system decisions, even for SOHO users. Cheap used 100 watt server boxes are no bargain, if you don’t need their capabilities – it is best to recycle them and save energy.

Keith Loftstrom