Ade Miller has a post on the idea of applying PUE to PCs. What is the total power used by a PC divided by power used by the PC motherboard and peripherals?

How Green is Your PC? Estimating Power Usage Effectiveness
Tuesday, May 12, 2009 – 3:00 am
How efficient is your PC? Here’s a thought… How about taking some ideas from the people who run data centers? Turns out data centers use a measure called Power Usage Effectiveness (PUE) to assess how energy efficient they are:

Power usage effectiveness (PUE) is a metric used to determine the energy efficiency of a data center. PUE is determined by dividing the amount of power entering a data center by the power used to run the computer infrastructure within it. PUE is therefore expressed as a ratio, with overall efficiency improving as the quotient decreases toward 1.
- SearchDataCenter.com

Err… So what does that mean for your PC? What if you think about the PC on your desk as a mini data center. How would you work out it’s PUE?

This brings up an interesting concept of publishing a PUE for IT equipment. Where is the power overhead going?

Unfortunately not all of the power drawn by your PC makes it to the parts that matter. Power supplies aren’t perfectly efficient. So the next thing to do is factor in the computer’s power supply (PSU) efficiency. You can probably get this from the manufacturers web site. If you’re lucky they’ll give you a graph showing the efficiency under different loads like the one on the right for my Corsair TX650W. If not pretty much all manufacturers will give you an average number, especially if its 80 Plus rated. If not you can take the average number quoted.

What’s interesting about the graph is it shows how it’s possible to use an efficient PSU inefficiently by mismatching it to the rest of the PC. The efficiency peaks and drops off for very low and very high power consumptions.

For example for my my developer/gaming machine the power reading under load is 240W, this is 240/650 = 37% of the PSUs maximum load. From the graph above this gives an efficiency of 83% for a 110V circuit so 240W * 83% = 199W is actually delivered by the PSU to the rest of the computer. Now you know the amount of power actually being delivered by the PSU.

Unfortunately some of the components inside your computer aren’t actually contributing to the bottom line, they’re just there to cool the bits that are. Namely… the fans.

Here are PUE numbers for Ade’s equipment.

I found this article on eweek on server fan efficiency.

Server fans: How to optimize the power used

Second only to the power supply, server fans have become a large user of power (other than the actual computing-related components themselves). As servers have become smaller and smaller—and now commonly pack several multi-core CPUs in a 1U-high server—the challenge of moving a sufficient amount of air through the server requires multiple small, high-velocity fans. They need to push air through very small restrictive airflow areas within the server and the very small intake and exhaust areas at the front and rear of the server chassis.

These fans can consume 10 to 15 percent or more of the total power drawn by the server. And since the fans are DC, they draw power from the power supply, thus increasing the input power to the server, again multiplied by the inefficiency of the power supply. In addition, in 1U servers, most or all of the airflow is routed through the power supply fans since there is virtually little or no free area on the rear panel to exhaust the hot air.

To improve efficiency, many new servers have thermostatically-controlled fans which raise the fan speed as more airflow is needed to cool the server. This is an improvement over the old method of fixed-speed server fans that run maximum speed all the time. However, these variable speed fans still require a lot of energy as internal heat loads rise and/or input air temperature rises.

For example, if the server's internal CPUs and other computing-related components draw 250 to 350 watts from the power supply, the fans may require 30 to 75 watts to keep enough air moving through the server. This results in the overall increase in server power draw as heat density and air temperature rises in the data center. In fact, studies that have measured and plotted fan energy usage versus server power and inlet air temperatures show some very steep, fan-related power curves in temperature-controlled fans of small servers.