2008 BMW Other electricial problem

The car will sometimes have no power at all when you go to start it, like the battery was disconnected. The door locks don't work, no lights, no power to anything. This usually happens when the air conditioner was used prior to parking the car. The car will power up after about 6 to 8 hours after you try to start it when there is no power at all. It has been checked at the garage and they found no problems with it. Do you have any ideas that what could cause this. This is my sons car and he is at wits end with it.
Your help would be greatly appreciated. Jim