After posting this conversation, and trying many different options, this is what works for me (Europe based G1000 ac)
a) Backup: a G496 which it is always on and in a complete electrical failure should provide immediate information. After facing a difficult situation, I would say that in a real emergency, the chance of looking for equipment around the cockpit would have a high correlation to losing control of the airplane. So the 496 is right seat yoke mounted and always working for you.
b) Flight planning, WX, plates, etc : A $370 Toshiba netbook which is able to run Jeppessen Software, and has a 12 hr battery. It can also be used in flight should you need an unexpected plate you did not carry onboard. Should Jeppessen decide to face reality and program Mac code, the Ipad would be hard to match.
Your goals are a bit confusing... are you looking for a back up to fly the plane with, or just a back up for the data... charts, plates, etc.
If you're looking to fly the plane after a total electrical failure, I'd use the standby "steam" gauges that would be right in front of me. For nav, the 496 would be fine and would save the day.
Now, for the charts and plates, my choice would be the IPad, but not sure what's available for your area. There's already two vendors in the US and that will probably increase. I'm headed to my IPad store as soon as I can find them in stock.
For now, I use an old Motion 1300, Windows computer, which has been just OK, but has the potential of high failure with Windows. It failed on a recent trip out west, however, I have enough redundancy in the panel to finish the job safely, including a 396.
Once I get the Windows operating system out of the cockpit, I'll be happy.... heading that way fast.