A cloud-based energy-saving service system and method are disclosed. The service system and method of this invention use the strong computation capabilities of the cloud computing technology and systems to determine the power consumption policies of the mobile devices and provide the policies to the mobile devices, whereby the mobile devices implement the power consumption policies to save its power consumptions. In one aspect, the invented system and method calculate and determine the backlight level polices for a particular video stream, then provide the policies information to the mobile devices for implementation. The policies information may be provided to the mobile devices independently or along with the video stream. The calculation of the backlight illumination levels may be done in advance or on remand.