I believe not much lighting left. If you look on the QRH for Emergency Elec config, there is a table showing the system remaining on battery only.
It happened to me to shut down the engines before the APU was available. cockpit went dark and very quiet for about 10/15 seconds until APU took over. Not a big deal in that case.
If you have already prepared the FMS, the data will still be there. One important thing in case you lose APU generator or EPU is to switch off the ADIRS. There is an external horn that will trigger until the ADIRS are switched off. The reason is that on battery only you just have partial avionic ventilation system working and the ADIRS system can overheat.