I have been watching with amusement as the data center owners and engineering firms fall over themselves claiming to have the "Lowest PUE/Most energy efficient" data center. It started with hardware manufacturers and software companies claiming they were designing their data centers for PUE's below 1.3. This was largely scoffed at since none of the details were provided to substantiate their claims. When they were finally provided, a number of very competent engineers questioned the validity of the numbers. I won't get into the arguments about how they were calculated, but I do think that this is a perfect example of why calculated or estimated PUE numbers are meaningless for anything more than a design goal.
Estimated PUE's are usually based on best case operating conditions. They are typically calculated based on the design load of the data center. In actuality, it might be 3 to 4 years before the loads in the data center reach the design loads. During this time the efficiency of the electrical and mechanical systems might be significantly lower than they would be at full design load. A UPS system that is 94% efficient at 90% load might be 80% efficient at 20%. The same concept is true for chillers as well. Yet in reality, the first three years of operation, 20 to 30% loads might be closer to the truth than 90%.
If you are using outside air for a large part of the cooling, the calculations typically assume that you will be on outside air 100% of the time that it is available. That is probably not even close to being achievable, since the realities of operating a data center may prevent you from always using the most energy efficient means of cooling. There may be extreme weather conditions that require you to operate with the outside air dampers closed. Since almost all of the low PUE data centers use some form of air side or water side economizer to achieve their numbers, not operating in that mode can have a significant impact on the actual PUE numbers.
A more meaningful calculation would be to measure the PUE on an annualized basis after the facility is on line and at its design load. This measured PUE would be much more real world than the estimated PUE that was calculated during the design process. This is not always politically correct, since this may not occur for years. Can you imagine the conversation you would have with a marketing person about waiting for 3 to 4 years to announce that you have the "worlds most energy efficient data center".
Having said that, I don't think that estimate PUE's aren't valuable. Having an idea of how your new data center might perform could affect some of the decisions made in the design process. An important part of establishing the owner's project requirements document is to have a design goal for PUE. I'm just saying that we should be realistic about how achievable calculated PUE's actually are. If we use them, we should always qualify them by adding that this is an estimate and that the actual PUE achieved would depend on a number of operating factors that might be beyond the control of the design engineers.