Always interesting to see how we can digress in our answers and I am as guilty as the next person!
How long will my battery last you ask? Find out the ampere hour rating of your battery, add up the amperage draw of the components you have on and the answer becomes a simple arithmatic question. That of course assumes a battery at 100% charge and effciency at the outset. That would also not ensure that you can energize high draw items such as a starter much below battery peak. So many other factors come into play i.e. how much cranking of the starter is going to be required to start the bloody thing due to pilot technique, engine condition, ambient temperatures etc.
It is a good exercise though to have an idea of the amperage draw of all various systems on your aircraft. On that dark and stormy night when the gods are conspiring against you and you find yourself in an emergency and down to battery power you will be very glad that you have gained that knowledge. You can be surprised that the one item that you thought would be a good idea to get you to some tarmac uses much more power then you had thought and you in fact might be better off using two of something else.
As an aside, I too was pleased to see that you refered to using the GPS as a backup to other items. It indicated to me that you were using the resources available to you whether it be a full blown FMS system or a map. Over the years as I have flown with some new hires I have sometimes seen an over reliance on the whiz bang gadgets that have entered our cockpits. Don't get me wrong I use them and enjoy all of them as much as any but don't forget the basics. When I look over at their side of the office and see that not all is as it should be and I reach down and make the FMS readouts suddenly go dim and then look at him/her and say "ok, where the hell are we and how do we get there now." it drives the lesson home....use all resources available to you.
Also, if you are going to go out ahead of time and start one fan to provide power while you program things might I suggest you taking an extra pair of eyes with you. You will have your head down and those eyes could keep a lookout for things such as your a/c moving, vehicles, stray people etc. I have seen the consquences of someone not doing that and it's not pretty.
Regards,
604guy