Which aircraft are we talking about?
Regarding the cabin. The Standby Busses and Battery Busses should power passenger address for announcements. The emergency lights should activate in the cabin (using their own battery power supplies). If you don't wish to flatten the emergency light batteries, switch them off (assuming there is enough light in the cabin for proper visibility). If power loss occurs just before a flight, note the time that the emergency lights have been in use. Our manuals state that the batteries must have been recharging for a certain amount of time before flight.