Flight Instructor reported total electrical system and engine malfunction during taxi to the runway for departure. The flight was cancelled and the plane was towed back to the FBO.

Narrative

Left the ZZZ [airport] FBO on a discovery flight; just after crossing [runway] XX on [taxiway] 1 the right G3X flickered off along with the Garmin 650 and a low drumming sound took over the comms. They then came back on and the low drumming sound stopped. I told the student and passenger that we would not be flying today and I had the goal of doing a 180 on [taxiway] 2 and 1 then taxiing back to the FBO. However; the electronic system completely failed just moments after the right G3X and the 650 had come back on. The G5 also failed and the drumming returned. The engine then failed and started chugging; but not turning as it sounded like the mags were trying to turn. There was the smell of electrical equipment burning; so I turned master off; electronics off; pulled out the mixture; turned the fuel selector valve to off; immediately had the passenger exit the airplane from behind the student; then had the student exit the plane. They returned to the FBO I secured the plane and it was towed back.