I have never used or closely looked at 3PAR so I can't speak to it, but I would say this for XIV....
XIV does have 243 TB, so approximately 50% usable vs. raw. That is due to it using the same mirroring algorithm on all data instead of traditional RAID stripes, like 5 and 6. Could you get better utilization by manually RAID striping data and carving out arrays in the traditional approach? Sure, but that has limitations at high scale and high variability. There is no way a "cloud" provider is going to be able to carve RAID out for every host at the outset, so a lot of there will be a lot of stranded space. Even if you could create a RAID plan for a high variable workload set, you would have to hire a bunch of people to manage storage which would not be necessary with XIV's approach. While people can theoretically do better than 50% utilization, very few do better than that in practice because it requires a lot of knowledge about what the storage environment will look like 2 years down the road when you set it up and constant management. VMAX, for instance, is supposed to have a real world average of about 20% utilization or something in that range... can't remember the exact figure but less than 50%. Even if you can get slightly better utilization, the costs of the staff will overcome any savings you might have of fewer modules of relatively inexpensive disk. You don't have to touch it with XIV or make any decisions... which, at scale, is not only good but almost required.
XIV can run at up to 8.4 Kw, max, but it is usually lower. XIV does have 15 CPU modules in a rack which I would think would be where a lot of the energy is used being that 7.2 drives don't pull much energy. Another system might require more arms to achieve the performance levels or peak hot spot avoidance of XIV's 15 CPU modules per rack which might require more than one rack in the other system to achieve apples to apples, so it is difficult to compare a rack to a rack for workload requirements because a rack of XIV is basically a server dense rack whereas other systems are primarily a bunch of drives.
"Could be why 3PAR is in 8 out of the top 10 service providers."
I imagine the top 10 service providers probably have a few of every major storage array in their data center. If 3PAR can claim that they have 200 PB under management in prod at one of those service providers, that would be interesting.