Modern OSes take steps to reduce the power usage of the system. Things such as throttling the CPU speed and putting the CPUs in to a halted state when they are not being used.
An article on Computer World points out that hypervisors today push the peddle to the metal on the host system whenever even one guest is running… causing the system to never enter a low power state.
While consolidating servers is a good environmental initiative, perhaps there is more room for the hypervisors to help cut the usage down per system.
This is a very interesting perspective on things. Virtualization has typically been regarded as supporting Green initiatives, not thwarting them… which I still hold to be true in most cases. Clearly there is a use case that doesn’t make environmental sense for a virtualization. I would consider this analogous to public mass-transit. If they are operating at a high capacity they are much more green than if each of those people drove their own car. However, if only 1 or 2 people is riding, the carbon footprint of that bus or train remains the same.