PUE: to compare is human to improve is divine. (part of the DS PUE series of articles)

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1 PUE: to compare is human to improve is divine (part of the DS PUE series of articles) Yes, I know it s an inadequate distortion of an old, clichéd proverb. Yet, I say this too often in client meetings and peer discussions, Don t compare the PUE (Power Usage Effectiveness) of your data centre to that of another because it s a pointless exercise. Remember this! The most important thing to remember from this article is this focus on improving your data centre s PUE. Don t be too concerned with the PUE values published by others because there s nothing you can do about that. Measure your data centre s PUE and use the metric as a baseline for improvement. Plan what you will do to improve and then, if it s feasible, execute the plan. Thereafter, measure it again. If the PUE value improves (i.e. reduces closer to 1.0 it can never be less than 1), then you re on the right track. Keep in mind that, as you gain a better understanding of PUE and the measurement thereof in your own environment, it may result in changes in your measuring methodology. This is okay as long as you follow the rules and do it in a correct scientific manner. The basics of PUE Power Usage Effectiveness (PUE) was introduced by The Green Grid in Since then PUE has become the most commonly used measurement metric to determine and demonstrate

2 how effective/efficient (or not) the use of power is within data centre environments. I m not going to bore you with in-depth details of PUE except for the general calculation formula itself: PUE x = Total Facility Energy IT Equipment Energy x = 1, 2, 3 (different levels of measurement) A PUE value that is higher than but as close to 1.0 as possible is desired but it cannot be less than 1. For example, Google releases their data centre PUE values quarterly and has been doing so for their entire fleet since mid The lowest TTM (Trailing Twelve Month) PUE for their fleet has been 1.12 and a value of 1.08 has been reported as lowest Quarterly PUE for any one of their data centres. It must be stated that Google aims to apply a higher standard or methodology of measurement for their PUE as compared to the looser interpretations that some may apply when using the PUE calculation and methodologies documented by The Green Grid/ASHRAE paper. Facebook reports their data centre PUE values in a more real-time manner as compared to Google. For this publication I m not going to dwell on who s more precise, more reliable or applies a better methodology in their PUE values because that would lead me to compare, which is exactly what I want to avoid. Personally, if your data centre has a measured TTM PUE greater than 2.5 (even on the African continent), then I d regard that as being highly inefficient. This is my opinion and nobody else s. Self-reported industry averages are now at the 1.9 level. I will refrain to comment as to how accurate I believe this value is though. Where else to find more detail on science of the PUE metric itself There are many articles and publications available that delve much deeper into the science and calculation of PUE. There are also different types of PUE. There s PUE for dedicated buildings, PUE for a data centre in a mixed-use facility and Partial PUE (ppue). There are also different levels of PUE measurement, methodologies and reporting as well Basic (Level-1), Intermediate (Level-2) and Advanced (Level-3). If you want to know more about the intricacies of PUE, the publication, PUE TM : A comprehensive examination of the metric, should satisfy your curiosity. The document is part of the ASHRAE Datacom Series. It was published in collaboration with The Green Grid (TGG). It can be downloaded for free from The Green Grid website but you must at least be an individual member. However, becoming an individual member of The Green Grid, so you

3 can gain access to this publication and a lot of other information for free, will set you back a couple of hundred US Dollars. Money well spent in my view. PUE refers to the effective use of power in a data centre nothing else Power Usage Effectiveness states how effectively the power is being utilised within a data centre. It discloses the data centre s performance only in terms of power usage. It does not tell you how well the IT equipment itself is providing usable output-data within the compute or I/O cycle. PUE does not measure the efficient performance of a data centre s end purpose, which is to derive and deliver usable digital output. For example, a data centre can efficiently provide power to a server all day long but the server itself may still produce no valuable or usable digital output whatsoever. The impact of PUE on the operational cost of electricity As an example, let s assume a data centre is configured to run in a certain manner with the current IT equipment deployed. Upon measurement the current PUE of this data centre is 3.0. Simplistically, this means that 2 Watt of additional power is required to support every single Watt that is directly used for the IT equipment (referred to as productive power ). The supporting power is referred to as wasted power. Any value that exceeds the value of 1.0 is regarded as waste. Calculation of a data centre that runs at a PUE of 3.0 Input assumptions: Server quantity: 300 servers Power consumption: 400 Watt per server Cost of electricity: USD 0.15 per kw/hr Hours per year: 8,760 PUE: 3.0 Total IT equipment load: 300 x 400 = 120,000 Watt = 120kW Total electricity cost per year: PUE = Total Facility Power Total IT Equipment Power 3.0 = Total Facility Power 120,000 = 360, ,000 Cost = 360kW x 8,760 x USD 0.15 = USD 473,040

4 I concede that the example above could be regarded as an over-simplistic approach to a complex issue but bear with me as we journey through this subject because I trust that things will be clearer and better understood at the end. Improved PUE could reduce operational costs A reduced PUE implies reduced total power usage, which results in reduced operational expense (i.e. the cost of electricity). A reduced PUE illustrates that less of the total incoming power into the data centre is required for the supporting load (Switchgear, UPS, Chillers, CRACs, lighting, etc.) as opposed to power required for the actual IT equipment (Servers, Storage, Telecoms kit, etc.). A reduced PUE implies that the efficient use and distribution of power within a data centre is improved. By default, most data centre owners and operators strive to see a reduction in their electricity bill as they improve their PUE. So there is a potential to save on operating costs in the medium to long term and also to lessen the consequential impact on the environment. This at least, in my view, makes the reduction of PUE worth considering for any data centre environment. Let s assume that you have, over a period of time, closed the air leakage holes on the data centre floor, deployed blanking panels in all the rack cabinets, installed better and more efficient cooling technology and deployed other support equipment that also runs more efficiently on any given day. As a result the data centre PUE has dropped to 1.5. This implies that 0.5 Watt of power is wasted on the supporting load for the same 1 Watt of productive power that is required by the IT equipment. Subsequently, if the IT equipment usage is the same quantity of power, it means that less power is needed for the wasted remainder. Therefore, all things being equal, a saving on the cost of power is realised. Consequently the operational cost of electricity will look as follows: Calculation of a data centre that runs at a PUE of 1.5 Changed input assumptions: PUE: 1.5 Total electricity cost per year: PUE = Total Facility Power Total IT Equipment Power 1.5 = Total Facility Power 120,000 = 180, ,000 Cost = 180kW x 8,760 x USD 0.15 = USD 236,520 Therefore a PUE of 1.5 can benefit by halving the annual electricity bill for this data centre. Again, this example and the calculation is very simple but the theory holds firm in terms of potential reduction in operating cost, especially when it comes to electricity usage.

5 There s more to come In the next article on this subject, we look at the approach to reduce PUE, the implications of reducing a data centre s PUE and what is required to embark on the road to an improved PUE. Author credentials Lee Smith is a specialist data centre design and operations consultant. He has spent the majority of his professional life in and around data centres. His experience ranges from IT operations and data centre design to infrastructure management and software engineering. He is passionate about the pursuit of effective and efficient design, operational excellence and quality service delivery in data centres. He is well versed in data centre standards and appreciates the importance of following good- and best practices. He can be contacted directly at lee@ds.co.za.