I’ve noticed in the new update that the screens will only turn on when the APU has been turned on. Although this is (sort of) realistic, in that not all screens will turn on while solely on battery power, it is also not realistic for the APU to be started right from when the pilot starts up the aircraft.
Although it may be started for purposes of air conditioning early, it is still not used for electrical power. In fact it is becoming increasing common for the use of APUs to be banned until just before push back, with air conditioning and power provided by ground carts.
In the absence of any ground services, I propose that the screens are turned on from battery power, as not only is this more realistic overall than requiring the APU, it also prevents having to listed to the APU for too long. (As nice as the new sounds are, the sound of an APU does get boring)