Phone Power Consumption 

The Cisco Unified IP Phone 7900 Series supports Cisco EnergyWise. EnergyWise is also known as Power  Save Plus. When your network contains an EnergyWise controller, you can configure these phones to sleep  (power down) and wake (power up) on a schedule to reduce your power consumption. The phone should be  powered by the Power Over Ethernet (PoE) port of the switch instead of the power adapter. 

You set up each phone to enable or disable the EnergyWise settings. You can also configure EnergyWise  parameters on the enterprise and common phone configuration. If EnergyWise is enabled, you configure a  sleep and wake time, as well as other parameters. These parameters are sent to the phone as part of the phone  configuration XML file. 

The switch administrator can wake the phone up before the scheduled time. For more information on powering up the phones from the switch, see the switch documentation.