Using Distributed Power Management (DPM)
The load of your virtual machines is probably not always the same. Sometimes they need more resources and sometimes they need fewer resources. Distributed Power Management (DPM) will use DRS to consolidate virtual machines on fewer ESXi hosts during periods of low resource utilization. The unused ESXi hosts will be powered off. If more resources are needed, DPM will power on ESXi hosts and DRS will move virtual machines to these hosts. DPM will save you or your company money in power and cooling costs.
Tip
VMware has created a fling called "Proactive DRS" that uses the historical data gathered by VMware vCenter Operations Manager to proactively power on hosts before a predicted increase in resource demands. You can find Proactive DRS at: http://labs.vmware.com/flings/proactive-drs
Enabling DPM
DPM has two modes: manual and automatic. In manual mode, DPM will give recommendations to evacuate virtual machines from hosts and power off or power on the hosts...