can't see how you could answer this question without knowing
And that is exactly why pilots don't get to have the worry about such calculations. The aircraft Electrical Load Analysis generally is done by electrical engineer specialists and this is covered in great detail in the design standards, other Regs, and advisory material such as AC 21-38, referred to above. Pilots generally don't get to see the ELA as it is done in the engineering/maintenance background as part of the CofA and continuing airworthiness maintenance. The pilot follows the POH requirements to do his/her part to maintain the ELA's operational integrity.
For VFR, the requirements are pretty straightforward as the aircraft should be operating in VMC. Usual deal, operationally, perhaps advise ATC, turn off the battery, and continue until you are approaching the destination/alternate and reselect the battery back on for essential things for the landing, as appropriate. The POH should give you the recommended story. Keep in mind that the aeroplane will fly fine without batteries operating - caveat, I never flew Airbus so I am not au fait with what the story is there but it will be analogous to an appropriate extent.
For IFR, in serious IMC, probably the most critical likely failure is a total electrical failure putting the aircraft back onto the battery. Generally, you can expect (with appropriate load shedding per the QRH) a bit over 30 minutes continued power to critical systems. That is to say, you need to get back on the ground within that time or else you will find yourself in a super world of REAL hurt. If you can't do that, the usual deal is like VFR - get VMC, turn off the batteries and proceed until approaching the destination/alternate etc.
One thing you can't do, IMC, is fly without any instruments - just doesn't work. Most serious IFR aircraft will have a basic standby flight instrument setup powered by a small, stand alone battery which helps a bit.
Main thing for the pilot to understand, VERY CLEARLY, is that electrical power is super critically important, especially at night and/or IMC. This is one of the problems associated with not letting your VFR-only equipped aircraft get out of VMC ...
It always was great fun, in sim endorsement/recurrency training on the B737 (for my background or any Type for that matter), to set up this sort of failure and see how the folk in the front seats managed. Most sims, at the 30 minute point ... just go totally black at which point the crew realise that they are dead and that their management of the emergency wasn't up to par. Bit of a discussion re strategies and then back for another go with a bit more crew thought as to how to get around the problem.
