What's a Watt Worth: The Cost of Energy

 
 
By Julius Neudorfer  |  Posted 2009-03-05 Print this article Print
 
 
 
 
 
 
 


What's a watt worth: The cost of energy

We don't always stop to think what it costs to operate a "small" server that typically can consume 500 watts of power. That server also needs to cool 500 watts of heat load (approximately 1,700 British thermal units or BTUs). So, for the typical data center that has a PUE of 2.0, that means that it uses one watt to support (power losses and cooling power) each watt of "plug power" to the IT load itself. This means that it takes 1,000 watts, or 1 kW, of power into the data center to run the 500-watt small server.

A single kW does not sound like much in a data center-until you begin to calculate that since it is consumed continuously (the proverbial 7x24x365), that it equals 8,760 kilowatt hours (kWh) per year! At 11.5 cents per kWh, one kW costs $1,000 per year (of course, 11.5 cents is just an average; in many areas the cost is much higher).

So, over a 3-year period, that one small 500-watt server can cost $3,000 or more just in energy costs. In fact, since many of these small servers cost less than $3,000, you can see why some analysts have predicted that the power to operate the server will exceed the cost of the server-especially as the cost of energy rises. Let's now examine where the power goes and what we can do to optimize it. 

Power supplies

The power supply is, of course, where the power enters the server and is converted from 120 to 240V AC to 3.3, 5 and 12V DC. Until just recently, the efficiency numbers were unpublished (and still only listed by some manufacturers). In fact, the EPA Energy Star Program, which mandated that all PCs have power supplies that were 80 percent efficient or greater, specifically exempted servers! This is one area where a few extra dollars spent to purchase a server with an 80 percent or greater efficiency rating can pay back large returns via the energy cost saving over the operational 3 to 5-year life of the server.

The difference between a 70 percent and an 87 percent efficient power supply will result in a 20 percent overall energy savings for the server power usage (assuming that same internal server load). This would also mean a similar range of overall energy reduction for the data center.

Moreover, these efficiency ratings are usually only provided at the power supply maximum-rated load. That does not reflect the actual operating loads that the server will be operating at in production. Typically, a server will be only drawing 30 to 50 percent of the maximum power supply rating (name plate), which means that the fixed losses in the power supply will result in less than the rated power supply efficiency value at full load.

Moreover, since we also want redundancy to improve uptime, we typically also order servers with redundant power supplies. These redundant power supplies normally load share the internal load, resulting in each power supply only supplying half of the actual load-which means that each power supply is only running at 13 to 25 percent of rated load. This means that the fixed losses are a greater percentage of the actual internal power being drawn by the internal server components.

When buying new servers, the lowest-cost unit may not be the best choice, even if the computing performance specifications are the same. So when specifying a new server, this is one of the best places to start saving energy. If the server vendor does not publish or cannot provide the efficiency of the power supply, think twice if the server really represents a good value.

In fact, if you are shopping for a large amount of servers, it would pay to invest in testing the actual total power drawn by the different manufacturers and models you are considering-specifically when loaded with your operating system and applications, both at idle and at full computing load. By spending an extra $50 to $100 on a more efficient server now, you may save several hundreds of dollars in total energy costs over the typical life of the server. Moreover, you may avoid having to upgrade your power and cooling infrastructure.

Another area that can save about two to three percent in energy usage is to operate the servers at 208V or 240V instead of 120V, since the power supplies are more efficient at the higher voltage (as well as the power distribution system).

 



 
 
 
 
 
 
 
 
 
 
 

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