Goddamnslacker
A valid scenario...one Engine driven generator inop so as per the MEL permissible to operate supposely ETOPS with the APU running...
Aircraft dispatches...APU Battery fails - APU will shutdown due no APU battery on the APU Hot Batt Bus.
Left with one engine generator supplying the whole aircraft plus a RAT which only supplies limited power….
Each engine has two generators, so if you depart with one engine generator MEL'd, you have three other engine generators available. Should you lose the APU battery and APU, you still have three engine generators available to provide power. Even if you should lose the engine with two good generators and the APU, you still have one engine generator available and it can carry the load.