As a one-time member of The Green Grid Technical Committee, let me summarize and correct a few points:
- The EPA has said that data centers use around 4% of US power. The federal government uses going on half of that, IIRC.
- Historically PUEs of 2.4 were common. It makes sense. In a closed building, it takes 1 unit of cooling energy to cool the heat produced by one unit of computation etc. The cooling systems were say 80% efficient so that makes 1.25 units of cooling energy. Add in power conditioning and UPS losses, and you easily get 1.4 to 1.6 units of non-computational work for every unit of computation, for a PUE of 2.4 to 2.6.
- A PUE of less than 1.0 is by definition impossible. Nevertheless, a few deluded individuals have reported them.
- In anything less than ideal conditions, getting the PUE below 1.2 is very difficult. A common mistake is to allow equipment fans to do some of the work of the heat exhaust fans. This effectively transfers facility load to the equipment and results in artificially low numbers.
- Low PUEs are harder to achieve in high-resiliency conditions. The large server farms used by MS and Google do not require the level of availability that enterprise data centers commonly do. These companies have the luxury of trading off equipment failure rates with power costs.
- Uninformed data center managers sometimes think they can use unconditioned outdoor air as a coolant. This is ill advised. High humidity levels, high sulfur and other contaminant levels, and particulates can cause premature equipment failure, not to mention voiding warranties. I visited a data center in a hot and humid location once that had had 40% disk drive failure in a year. It's best to consult a professional data center HVAC specialist with experience in low-PUE installations.
- Power costs do sometimes exceed CAPEX over the life of the equipment. It depends on location and up-front equipment cost.
- Power gotten from public hydro is not "green". Power in the public grid is a zero-sum game. Only renewable energy that you produce yourself is more green than the average greenness of the public utility system.
- The data centers located near big dams don't get cheap power because they're being green. They get it cheap because they avoid grid distribution costs. The same thing can be accomplished by colocation with a big coal plant. It just doesn't sound as cool.