Looks at how replacing desktops with thin clients can save money and power for an organization. The post builds its case using step by step comparison of the computing resources usage scenarios and how these would layout in a thin client deployment versus a desktop deployment of a 5,000 workstation scenario.

by Steve Carl, Senior Technologist and Green IT Spokesperson for BMC Software. Read his blog, Green IT. Follow him on Twitter @stevecarl.

In my last Green IT post “Low Hanging Virtualization Fruit” I mentioned that I thought VDI would be a great way to save power and therefore reduce CO2. I thought it would be interesting to walk that intuition though some actual assumptions (similar to what I did in “Watts Up” for virtualization)

It all starts with the idea of what the average number of computers is that a single person uses professionally. I know I am way above the curve on that one, with four, but I am not that unusual for an R&D person. High average users are balanced by the mobile users that have one or two systems, and one of those is a laptop.

So, here is assumption number one. My typical person in this model will have two computers. One desktop, one laptop. The desktop will stay at their primary workplace (office or home) and the laptop will follow them around wherever they are. In today’s world it is also possible that the laptop is really a tablet computer such as a Motorola Xoom or Apple iPad.

Since this is about VDI, I am going to assume that the laptop / tablet will be one of the VDI access devices, and not be replaced, and therefore not considered for power reductions. As a power consumer, a laptop is already less than a desktop. My M4500 has a 130 watt power supply, and I have run it off a 70 watt unit. My Macbook has a 65 watt power supply, and so uses probably 50 or less.

Just the Desktop Then

The desktop in the model will have a 275 watt power supply (like my Dell 745 does..), and on average use, with a graphics cards that can drive dual monitors I will assume the data center diversity factor planning standard of .6, or 165 watts when in use.

I will also assume that the desktop has two monitors, and even go so far as to assume that they are at least LCD of some kind. My E197FP’s use 40 watts each, but they have Cold Cathode backlights. A quick look at the current model U2211H shows it using almost half that, at 22 watts. I presume most of that difference to be the LED backlight technology. So, next assumption: Half the population will have LED, half CC, or on average about 30 watts per screen. Times two, for 60 watts total. Add this to the desktop number and we have 225 watts per person per desktop. Assume older PC’s with CRT’s and this number gets much bigger very quickly.

Cross check: I found this table over at the University of Pennsylvania web site, and it quotes the 745 as using 145 watts to boot, and then using between 111 and 133 watts when active, but this is in a single monitor configuration and includes a low power monitor, so I think my 165 number is probably OK. Maybe a little high, but I’ll be extremely conservative estimate-wise in other areas so I think I’ll run with this.

This is that “while being used number”. When in standby, both the screen and the desktops use far less power (2 watts each, both panels and desktop computers, for a sleeping total of 6 watts)

How many hours a week is it using 225 watts and how many only 6 watts? I don’t know anyone that puts in only 40 hours a week, and I also don’t know many people that let their desktop system go to sleep. Their laptops? Sure. Their desktops? Hardly ever as near as I can tell. Some people still run screen savers… on LCD’s. I let the monitors go to sleep of course, but to be able to remotely access the desktops they can not be asleep. The uPenn data says that idle the desktop uses 111 watts, and that includes the Dell Ultrasharp monitor at 2 watts. Round number, lets just postulate that idle it probably uses 110 watts. And some people in the populace will let the desktop sleep, so that will bring the average down a bit. Call it 100 watts inactive then. Adjust this number whichever way needed to account for policies that enforce sleeping when inactive on a desktop. Since a fully hibernated desktop is using about 2 watts, if the majority of the PC’s are set this way, it hugely changes the assumptions.

Another thing that would influence the assumptions is the general age of the desktop. My Dell 745 is middling old at about 4 – 5 years. The older the desktops you are looking to replace, the more power they will use, the less well they will sleep when idle.

How many desktops am I after here? We have more than 6200 people here: Lets see what replacing 5000 desktops with a thin client looks like.

Thin Client

How much power does the desktop replacement thin client use? I looked at the specs of the Wyse V50LE and it appears to use 15 watts when connected to 2 monitors. A quick survey of other options makes that appear fairly typical for a dual monitor thin client, so we’ll go with that. The monitors would be the same as they were before: I’ll assume that most of the LCD’s are just moved over, that there are still two of them, and so we are using 60 watts when active and 4 when not for the screens. That is about 75 watts when active, and 6 watts when not active. The not-active wattage is a guess for the thin client,as it is not specified,so I assume that it is the same as the desktop. It is probably less, but trying to go worst case here.

Disk Space

How much stuff is on the average desktop hard drive? How big does the internal disk really need to be? I imagine that this, more than most anything, is widely variable. There are pack rats that keep everything ever written since the dawn of time, and those people that keep asking me for a copy of something I sent them last week, because they already deleted it. I try not to take it personally: They do that to everyone.

My fully patched Windows XP guest has a 10GB disk, and is full at all times. My Win7 desktop has 80GB in use. My Win7 guest (oddly) has 98GB in use. Win7 looks like it uses about 9 or 10 times what XP does. For the model I’ll assume that the virtual disk is 100GB, but that 20GB of that is not unique, so that only 80GB needs to be considered for end user space per desktop. This is probably high, but keeps me firmly on the conservative estimate side of things.

5000 times 80GB is 400 Terabytes. Wow. We have a ton of capacity at the edge of the networks.That number is probably high, and with hardware thin provisioning, virtual storage, in storage data de-dup, etc, there are all kinds of ways to reduce that footprint. Once again, to try and paint this as black as possible, what is the power for 400 TB? While we are at it, lets make this usable TB, and account for RAID and hot sparing of disks.

My sample design for this will be Dell Equalogic. A PS6500E, PS6500X, and PS6500XV. Tiered together so that hot data resides on the 15K disks of the XV, medium access data on the 10K disks of the X, and lightly referenced data on the mass storage of the E. Take 80% of each ones capacity, and rounding down, that is 105TB total, and uses about 2200 watts. 21 watts per TB, 8,400 watts for 400TB.

This is probably way high. There are all sorts of solutions inside VDI to save disk space, not even counting what modern storage can do. Virtual Bridges VERDE’s use of local disks where available for example. VMware’s Link Clones. Xen’s Provisioning Server, etc.

Nothing Is Free: A Host

Of course, the thin client works because there is a host someplace in a glass house / internal / external cloud running the actual virtual desktop. On this server there are bunches of other virtual desktops similar to the one running here. Similar, but not the same. In our experimentation with VDI, we see that that things like the memory reuse/overcommit were lower than if we were running a bunch of similar servers on the same host. On the other hand, a desktop OS does not typically use as much RAM as a server OS.

My typical desktop OS assumption will be a 2GB with a single CPU. History tells me that using dual CPU in a virtual environment only makes sense if the VM has a history of using more than one real processor a great deal of the time. If it does not, then the overhead of dispatching the second virtual processor is actually a performance decrease rather than increase.

This is a fairly large VM: My desktop running Win7 only has 2GB, and runs fairly well most of the time. Windows XP can not even use 4GB without being 64 bit, due to some oddities in the way the memory management and other historic design issues work. I actually use my 2GB desktop as a virtual desktop when I am on the road, and it works quite well, so I think this is a good average design point for this model.

How many VDI’s per host then? A survey of the sizing data out there from various vendors is pretty widely disparate on that number. It is also clear that there are heavy users and light users (See this VMware doc and this one from Parallels), and that unlike a typical server virtualization engagement where RAM is the first virtual host bottleneck, CPU and I/O seem to be the places that the hosts get hung up for virtual desktop. Going dense on the RAM probably means spending some time optimizing the I/O subsystem to be sure that the host can handle the maximum number of virtual desktops.

Then there is the way that the VDI solutions architecture influences that variables, such as Virtual Bridges Leaf and Branch design.

Clearly how many hosts it will take is going to be my loosest part of this estimate. Still, looking at the literature, and thinking about my general observations of the current VMware data from our BCO tool, I am going to drive a stake in the ground and say that a Dell R810 with 256GB of RAM and an optimized I/O subsystem should be able to handle a mix of heavy and light users of around 200. That may be high: That may be low. I admit, its something that would need a lot of validation, and would really depend on what the VDI solution is, and how you implement it, and how many people are remote. That means we’d need about 25 hosts though, and at about 1100 watts / host. That is the nameplate, so about 660 watts steady state after booting.

Related post: “The Way of the Green Code“, proposes that software design practices need to change in order to produce software that is both greener itself and that is designed to help other systems and products become greener.

Power Costs and CO2 Emissions

How you power is generated, and where you are in the world directly impacts how the electrons are being pumped to your data center as well as how much you pay for that electron pressure. Here is some data for four locations we have data centers that I will use:

State Tons / KWHr Cost per Kwhr
California 0.4942 0.1248
Arizona 1.149 0.0911
Texas 1.385 0.1099
Massachusetts 1.184 0.1627

These are averages, and only useful for comparison. For example, Austin Texas is working on moving 25% of the city power to wind generated over the next few years, so a DC in Austin will have a lower CO2 footprint than a DC in, say, Dallas or Houston or Amarillo. Yes: They have things other than cows in Amarillo.

The Numbers

That should line up all the assumptions for the model. I’ll assume this is being studied over a three year period, since that is the shortest amount of time anyone would probably keep a desktop before replacing it. The reality is probably better than that, given that the average PC life-cycle has lengthened.

Power of the Desktop


Number of Desktops 5,000
Watts Per Desktop (active) 225
Kwatts per Hour total 1,125
Kwatt hours / week 39,375
Watts per Desktop (inactive) 6
Kwatts per Hour total (inactive) 30
Kwatt hours / week (inactive) 3,990
Hours per week (active) 35
Hours per week (inactive) 133
Total Kwatt/Hrs week 5,115
Total Kwatt/Hrs year 265,980
Total Kwatt/Hrs 3 years 797,940



Power of the Thin Client


Number of Thin Clients 5000
Watts per Thin Client (active) 75
Kwatts per Hour total 375
Kwatt hours / week 13,125
Watts per TC (inactive) 6
Kwatts per Hour total (inactive) 30
Kwatt hours / week (inactive) 3,990
Hours per week (active) 35
Hours per week (inactive) 133
Total Kwatt/Hrs week 4,365
Total Kwatt/Hrs year 226,980
Total Kwatt/Hrs 3 years 680,940


External SAN Storage Power for VDI hosts


400 TB @ 21 watts / TB 8,400
Hours per week  168
Total Kwatt/Hrs week 1,411
Total Kwatt/Hrs year 73,382
Total Kwatt/Hrs 3 years 220,147


Here is where it all comes together: How much money in power, and how much CO2 can be saved in this model, VDI over desktop?


California Arizona Texas Massachusetts
Tons CO2 USD Tons CO2 USD Tons CO2 USD Tons CO2 USD
3 yrs Desktop 5,213,563 $1,316,578 12,121,376 $961,059 14,611,058 $1,159,390 12,490,608 $1,716,404
3 yrs Thin Client 336,521 $84,981 782,400 $62,034 943,102 $74,835 806,233 $110,789
3 years VDI hosts 209,892 $53,004 487,992 $38,691 588,223 $46,676 502,857 $69,100
3 years storage 108,797 $27,474 252,949 $20,055 304,904 $24,194 260,654 $35,818
Total VDI solution 655,209 $165,459 1,523,341 $120,780 1,836,229 $145,705 1,569,744 $215,707
VDI Savings over Desktop 4,558,354 $1,151,118 10,598,035 $840,279 12,774,828 $1,013,685 10,920,864 $1,500,696

{Slightly updated: Found spreadsheet error on VDI hosts for MA. Fixed. Reuploaded SS}

One thing is for sure: I don’t think I’d try to roll this out to 5000 people all at once. The assumptions here are large. The design / vendor choices influence this hugely. For fun I messed around with this model seeing how more and less efficient desktops affected the bottom line, and the answer of course was “in a big way”. All my numbers are here, and I attached the OpenDoc format spreadsheet if you want to plug in your own numbers to see how your assumptions move the data around.

Finally, there are a bunch of good reasons other than being Green to use VDI. Moving your desktops inside your data center is a big one. it is much easier to back up your data assets when they are inside the glass house / internal cloud. Security is of course huge: Lost laptops are not nearly as upsetting if by losing it, all the data stayed right where it was, and nothing went missing. Being able to pick up, move from device to device, place to place, and resume where you were in your work is a nice productivity enhancer.

See related post: “Top 10 Things Data Centers Forget About PUE“, for a discussion that points out ten areas of power usage that are not being captured in PUE measurements; some of which have significant implications for PUE measurements.

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© 2011, Steve Carl. All rights reserved. Do not republish.

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Author: Steve Carl (2 Articles)

Steve Carl has been working in the IT industry in a wide variety of roles for three decades. Currently he is Senior Technologist and Green IT Spokesperson for BMC Software, where he has worked for the last 22 years. He works in a wide variety of roles, including data center design and implementation, technology solutions for R&D, Performance and Capacity Planning (including not just traditional performance metrics but also thermal / power performance). He is the author of two customer facing blogs for BMC: “Adventures in Linux”, where he chronicles his work in both data center and desktop Linux use in the Enterprise, and “Green IT, which covers his work in BMC’s green datacenter efforts. Follow him on Twitter @stevecarl