Anandtech research concludes:
1. Low power L-series Xeons save power but do not save energy in a typical Hyper-v virtual consolidation ratio. Power is “capped”, but the total energy consumed for a certain task is (more or less) the same.
2. High Power X-series Xeons offer a much better performance per watt ratio, but at the expense of brief power peaks. They do not necessarily need more energy in the long run than the lower power versions, and offer much better response times if your application is CPU bound.
The configuration tested a single Xeon 5600 using a workload consisting of “one tile” of the vApus benchmark on each of tested servers. Such a tile consists of a OLAP database (4 vCPUs), an OLTP database (4 vCPUs) and two web VMs (2 vCPUs). So in total we have 12 virtual CPUs. These 12 virtual CPUs are much less than what a typical high-end dual CPU server can offer.
Source: Low Power Server CPUs: the energy saving choice? – AnandTech. Must reading for IT datacenter planners.