Air carrier First Officer reported a failure of the APU and electrical smoke/fumes in the cabin while at the gate. Passengers were deplaned.

Narrative

During preflight briefing; I started the APU and transfer over power and air conditioning from jetbridge. Shortly after concluding the briefing; the CA called for the preflight checklist. At this moment; when reaching for the checklist; I noticed a strong smell indicative of electrical wires overheating. I looked over to my right side to inspect the CB (Circuit Breaker) panel that sits there; and could see a white haze developing in the area. I voiced to the CA 'we have an electrical fire.' Fearing that the smoke may also be propagating into the main cabin; I instinctively turned off both PACKS. At the same time; the CA called on the FA to investigate source of smoke -believing it may be coming from one of the ovens. Simultaneously; he noticed that the cabin was filling up with smoke. FA informed CA that they could hear a banging noise coming from the tail of the aircraft -at which point; we lost all AC power. The APU had turned off without indication of fire or any faults as would be displayed on EICAS. At this point; it was unclear whether an expedited deplaning would be warranted considering the smoke that was present or maybe even a full evacuation in the event we were actually on fire. Hence; the CA left the flight deck to gather more information to make the proper decision. Operating on DC battery power only; I was able to raise the ramp crew to ask if they could see signs of smoke or fire. They confirmed seeing no sign of such. I advised them to stay away from the electrical system and to keep jetbridge attached as we may be conducting an expedited deplaning shortly. Before CA stepped out of the flight deck; after the aircraft lost all AC power; the CA addressed the passengers via PA to acknowledge that there was a problem with the aircraft. Hence; asked them to listen up in case he commanded a deplaning. Therefore; when he returned to the flight deck; he informed me that due to the smoke in the cabin; it would be prudent to conduct an expedited deplaning. He got on the PA system and calmly directed the passengers to leave everything behind; and exit the aircraft via the main cabin door. The event from first sign of smell to losing the APU; may have taken around 2 to 3 minutes.