PUE, which measures how much power is actually used by computing equipment in contrast to cooling and other factors, has achieved broad adoption within the tech sector, and has value in helping datacenter operators benchmark design and performance of their facilities, Greenpeace pointed out.

"However, companies are happy to report this figure because it reveals nothing in terms of their actual energy consumption," the organization says in the report.

"It's a poor metric for determining how green a datacenter is, as it doesn't account for how firms manage computer resources inside the datacenter, and in some circumstances, it penalizes better performance," the report reads. "For example, if a cloud manager identifies servers that aren't in use and decides to shut them off and create virtual servers, that can result in a decrease in power consumption but an increase in the facility's PUE."

According to the organization, a PUE of 2 or more was average until recently, indicating a datacenter where nearly half of the demand is being directed to equipment other than the computers inside. By comparison, a "perfect" PUE score would be 1, indicating all electricity is being consumed by the computers, and utility-scale cloud companies are increasingly reporting PUEs between 1.1 and 1.6, Greenpeace noted.

Another severe limitation of PUE as an indicator of green performance is that the figure is unrelated to the carbon content of the electricity being consumed. "For example, if a datacenter is largely powered by renewable energy but has a poor PUE, it will still contribute significantly less pollution than a facility largely powered by coal but with a lower PUE," said Greenpeace.