B737-700 Captain reported a Master Caution light illuminated during the climb-out and after some reassessment; realized there was an entire system failure. The flight crew performed an air turnback.

Narrative

Preflight; we noted several previous write-ups of the #1 Electrical System. We opted to start both engines at the gate to ensure the #1 IDG (Integrated Drive Generator) came online correctly. All systems appeared to function normally. Climb-out; passing through the mid-teens; we had a Master Caution light illuminate with many systems illuminated on the recall. We analyzed the situation; initially thinking the light was the likely root cause of this situation; [and] quickly realizing that the issue was greater than just a TR (Transformer/Rectifier) failure. We started the QRH; we re-assessed the situation; and ran the QRH for a Left Generator failure. We noted that we did not have a Source Off light. Realizing that this was an entire system failure; based on a number of other system lights illuminated i.e.; 2 fuel pumps; a HYD ELEC PUMP; ANTI ICE; YD; etc.; we started the APU and got out of the QRH. The APU would not power the #1 Electrical Bus. I transferred control of the aircraft to the FO (First Officer) and I started coordinating with ZZZ Operations asking for a patch to Dispatch. FAs (Flight Attendant) called up and advised that they had absolutely no lights in the cabin. However; at some point during this event; the emergency lights did activate and give them some light in the cabin. Realizing that we lost ACARS during this even; we asked again for a patch to the Supervisor. Unable to get a patch; I spoke with ZZZ Local Maintenance on the radio and described what we had. Being that we were a single electrical system; and in agreement with their advice to return to ZZZ; we got priority handling into ZZZ. I made several PAs to the cabin; and several calls to the FAs to keep them informed on our intentions. I opted to use FMC-generated V/S and would use Autobrakes 3. On short final; the Autobrake Fail and Anti-Skid Fail lights illuminated. After landing; the APU and ground power would not power either side of the electrical system. Aircraft deplaning was done with emergency lights only. Post-flight; FAs were debriefed; and a conference call with Company Operations Center Chief Pilot; Manager on Call; Supervisor on Duty; and Dispatch was completed.