I've been on two chairs when the diesel backup had to be used. I'm assuming all chairs have some sort of backup system? I'm not sure I want to know the answer to that question.
I am pretty sure that in Canada, by law all chairlifts are required to have a backup small diesel evacuation motor. At Sun Peaks they have 3 chairs that have a 3rd motor that is a diesel capable of operating at 80% of the speed of the electric motor during a power outage.
A few years ago the electric motor on one of the main chairlifts out of the SP village failed. The motor had to be removed and shipped from B.C. to a company in Pennsylvania to get repaired. The motor was gone for a month and the chair operated during that time on the 80% capacity diesel motor.
2 weeks after the rebuilt electric was reinstalled it failed so it was shipped back to Pennsylvania (this time under warranty) and it was back to running again on diesel. Without that 3rd 80% capacity diesel motor the ski season would have been a disaster but with it there really was very little inconvenience for the skiing public. I think it helps that Nippon Cable, owners of Sun Peaks, are also principle owners of Dopplemayr.