A decade ago, PUE numbers were much more commonly closer to 2, (Google's were in the 1.2-1.3 range even in 2008, but things had improved quite a bit since 2005 already, and MSFT and YHOO were working on it), but modern datacenter design has improved that substantially.

see, e.g., https://www.google.com/about/datacenters/efficiency/internal...

In my understanding, 100% of the power consumed is converted to heat. Measuring efficiency should have a unit like Gigaflops per Megawatt or something, not a fictional percentage. That way you can also account for hardware getting more energy efficient over time, which is now just barely represented in the numbers. It's hard to get a right unit though, because for instance archive.org has a big data center but focuses on storage instead of processing power.

Gflops per watt is also really important, but that's more in Intel/and/Nvidia's hands.

Application ops/w is also also important, and that combines the system efficiency and the code efficiency.

A key thing is that each of these evolves on different time scales. When you build a datacenter it lasts for multiple generations of processors. Hence PUE.