B737-800
Flight Crew Air Transport Pilot (ATP)
Flight Crew Total 13000
Time Pressure; Situational Awareness; Human-Machine Interface
Aircraft Equipment Problem Critical
Flight Crew Diverted; General Declared Emergency
Aircraft
Legal placard of #1 Generator; APU operation required for back up generator per MEL. APU serviced prior to initial departure per logbook placard and an uneventful first leg was completed. On the next leg; about 45 minutes from destination the APU failed which resulted in a single electrical power source operation.  The QRH directs a landing at the nearest suitable airport. I deemed ZZZ the nearest suitable for landing; declared an emergency for priority handling and landed. I commend my First Officer for his outstanding assistance during this event. In the future; I will consider day/night and weather considerations before accepting an aircraft with a deferred generator. There should be weather minima associated with the placarding of a generator to prevent this from occurring again.
When dispatched with one generator inoperative the flight crew of a B737-800 was forced to divert to their nearest suitable airport when the APU; required to be operative to provide a secondary electrical power source; failed enroute.