Be aware that on departure, you have to contact Delivery first. Last time I was there, there was over a 40 minute wait just for start-up.
This wait obviously isn't too much of a problem if you're sitting in a 747 with the APU running. Bit more of a problem if you're trying to maintain some battery life. I suggest you remind the controller that you're on 'battery' and that you'll call him back at a certain time for start clearance.
HTH,
B73