That is abnormal. It should not lose more than 5-10 % overnight when it's at sleep mode, thus when it's off, it should conserve the battery even more.
It sounds like a good idea to have them have a look. Depending on where you are, if they need to do anything, an apple store can probably do things faster (like check the battery, change it if needed). Are there any Apple Stores close to where you live?