Thanks for your replies.
I asked because I was travelling on a saab 340, the aircraft made a quick (30mins?) stop enroute, after both engines were shut down half the cabin lights went out and things went awfully quiet. There was no GPU, yet everything in the cockpit remained on. Just wondered how long you could remain in this configuation for?
I mean, everything has to be 'on' to preflight and check the equipment, but how do you have enough battery power to power the avionics, and still turn both engines? I only fly small things were the engines are started about 2 mins after the battery comes on, then everything is done when the alternator is alive and kicking. Just curious, thanks!