Windows Desktop Manager uses as much battery as Mail! And it doesn't stop even when the device is on standby. Here is what happened to me: I changed the desktop background in desktop mode and picked a picture located in my SD card. I was under the impression that when you do that the picture is copied by windows and saved somewhere else. Later on I changed the configuration of the SD card to use a folder instead a letter. So the path to my picture was not anymore the same to the eyes of Windows. Let my Surface go to Standby during the night and when I tried to turn it on in the morning I couldn't. The Battery was completely drained. When I finally could turn it on after having connected to power for around 15 minutes I found that was the Windows Desktop Manager the cause.
There is not any reason why applications such as IE, Mail, Desktop Manager and in fact any other should be kept running when the device is in Standby. I understand that it's good to have all the emails retrieved when you switch on your computer but these are processes that take just seconds to performs. No a good reason for having your battery drained in normal conditions up to 7% in less than 9 hours or totally drained like it happened to me.
The Power Management in Windows 8 has been improved but still buggy, no ready to compete against the iPad.