Maybe this will help out as well.
Any 2 main generator sources will provide all power necessary for the aircraft. That being 2 IDGs or an IDG and APU.
The backup generators will come on line for only these reasons: 1. Testing after engine start 2. Only 1 generator including APU available, 3. Power to one or both main AC buses lost, 4. APP mode selected.
Normally when an aircraft is degraded to one available main system output, whether it be electrical or hydraulic, land at suitable airport will be the requirement.
There are certain requirements for dispatch under MEL for ETOPS and the MEL clearly defines what system components are required electrically. Unfortunately, I don't have my MEL book here as I write, so I can not be specific about the 120 / 180 rule. Suffice to say, the backup generators are there only for back up purposes and I believe dispatch with a back up alone and no APU will disallow ETOPS. It is there for strictly back up purposes.
The RAT is not a main source of power, in fact, the RAT will look after hydraulic requirements first before any electrical, thus, the system will shed electrical to provide hydraulic power for the aircraft components. The RAT provides 1/10th the power of an IDG.
Hope it helps