Data Centers

IT Cost Containment: The Power Benefits of Virtualization

In my previous posting, I talked about some of the direct hard cost benefits associated with adopting a virtualized server infrastructure.  A TechRepublic reader correctly pointed out that there are also a number of other costs benefits, direct and indirect, hard and soft, to be realized under a virtualization scenario.  In this posting, I'll talk about the power-based savings and how they might affect your organization's bottom line.

The Scenario

I'll use the same scenario used in the previous posting.  In that post, the fictional organization used orginally had 70 x86 servers all using direct-attached storage.  By the end of the post, the organization had fully transitioned to a virtualized environment for all but six of its servers.  Further, all storage, with the exception of boot disks for the operating system on each server, now resides on a trio of EMC AX4 storage arrays.

The hopeful end result

By the end of this post, I'll show you some rough cost savings related to eletricity.

I won't be able to speak much to cooling costs since the calculations would be somewhat prohibitive without more specific information, but do understand that, the fewer the servers, the less hot air that is blown out into the data center.


I'm going to make the following assumptions, some of which are based on the information in my previous posting:

  • The legacy server platform is a mix of Dell PowerEdge 2650's (10 units), Dell PowerEdge 2850's (10 units), and Dell PowerEdge 2950's (10 units).  Why this mix?  It seems logical that the fictional organization would purchase the latest server model each time there is a replacement.
  • I've used Dell's Datacenter Capacity Planner to calculate the power requirements that I'll use in my examples.  I've made the assumption that each legacy server has two 1GB DIMMs for a total of 2GB of RAM and 4 SCSI drives.  Finally, I've indicated that each server performs an average workload.  Also, each legacy server has direct-attached storage consisting of four SCSI disks.  Using these calculations, a legacy Dell PowerEdge 2650 uses 207.3 watts, which I'll round to 210 watts.  A PowerEdge 2850 server uses 348.2 watts, rounded to 350.  A legacy PowerEdge 2950 server uses 350 watts of power.  The figure assumes the use of normal voltage Xeon processors, not the low voltage variety.
  • The replacement servers are all Dell PowerEdge 2950's (revision III) — forever.  Since I don't know what Dell might release three years from now, all I have to use for calculation is what exists today.  I'm assuming that each server has two mirrored drives for booting the OS and 8GB of RAM.  Each PowerEdge 2950 server consumes 323.3 watts or power, which I'll round to 325 watts for the purposes of my calculations.
  • On the array side, the primary SAS array consumes a maximum of 450 watts of power while each expansion array uses 360 watts of power.  These figures are for SAS arrays, which is what I was able to find, but I'll apply them to the two SATA-based arrays, too.
  • As per the information located at, I'll use the November 2007 figure of 9.6 cents per kilowatt hour for commercial power in the United States.  I won't be accounting for inflation in electrical costs in this scenario.
  • Servers run 24/7/365.

The Calculations

With the assumptions in place, I'll start with some calculations:

  • Cost per year to run a single Dell PowerEdge 2650 server:
    • 210 watts x 24 hours per day x 365 / 1000 = 1,839.60 kilowatt hours.  1,839.60 x $0.09 = $165.56 per year
  • Cost per year to run a single Dell PowerEdge 2850 server:
    • 350 watts x 24 hours per day x 365 / 1000 = 1,839.60 kilowatt hours.  1,839.60 x $0.09 = $275.94 per year
  • Cost per year to run a single Dell PowerEdge 2950 (legacy) server:
    • 350 watts x 24 hours per day x 365 / 1000 = 1,839.60 kilowatt hours.  1,839.60 x $0.09 = $275.94 per year

So, prior to virtualization, the total power cost necessary to run the fictional data center is $7,174.40.

Now, let's look at what happens to power costs over a three year virtualization project:

  • Year 1: All PowerEdge 2650 servers are taken out of service.  Four PowerEdge 2950 (new rev III) servers replace these units.  Further, an EMC primary array (with controllers) is purchased as is an EMC expansion unit.  The four PowerEdge 2950 servers cost $1,024.92 to run.  Power for the two EMC arrays costs $559.76.  The remaining servers are still in operation and cost $5,321.70 to run. 
    • Total electricity cost: $6,906.38.  Not much savings, but remember that we've added the two arrays.
    • Total power savings: $268.02.
  • Year 2: This year, the PowerEdge 2850 servers are taken out of service.  Four PowerEdge 2950 (new rev III) servers are installed and an additional AX4 expansion unit is added.  Power costs for a total of eight new PowerEdge 2950 servers equals $2,049.84.  The AX4 installation costs $764.75 in power.  The remaining legacy servers cost $2,759.40 to run.
    • Total power costs: $5,573.99.
    • Savings this year: $1,600.41.
  • Year 3: Final year of this analysis.  The legacy PowerEdge 2950 servers are replaced.  Four new PE 2950 (rev III) servers are introduced.  All twelve PowerEdge 2950 servers cost $3074.76 to run.  No more storage arrays are added, so power costs stay at $764.75.
    • Total electrical costs: $3,829.51.
    • Savings over the original legacy environment: $3,334.89.

Total power savings over three years: $5,203.32.

Bear in mind that this figure does not include savings related to the need for less cooling power.


Since 1994, Scott Lowe has been providing technology solutions to a variety of organizations. After spending 10 years in multiple CIO roles, Scott is now an independent consultant, blogger, author, owner of The 1610 Group, and a Senior IT Executive w...

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