What is the minimum power system required to fly a commercial aircraft?
I heard of a A320 landing, no lights, both generators out, RAT deployed ... , made a successful landing. Is this possible?


Yes


  A United Airlines Airbus A320-200, registration N409UA performing flight UA-497 from New Orleans,LA to San Francisco,CA (USA) with 109 people on board, was in the initial climb when the crew reported smoke in the cockpit, levelled off at 5000 feet and returned to New Orleans. The crew reported before joining downwind that they had lost all instruments and requested to be talked down by ATC via Precision Approach Radar (PAR). The crew descended to 600 feet where they got visual contact with the water of Lake Pontchartrain and continued visually for a landing on runway 19 about 10 minutes after departure. During landing the aircraft blew both right hand main gear tyres, went left off the runway, stopped with all gear just off the paved surface north of the intersection with runway 10/28, and was evacuated via slides.
  
  A number of passengers needed medical attention due to smoke inhalation.
  
  Post landing photos showed the RAM Air Turbine (RAT) deployed. Runway 01/19 was closed for about 10 hours.
  
  The crew told passengers that they had lost all electronics and were flying on minimal backup systems, landing would occur overweight with minimal braking and minimal steering ability.


From http://avherald.com/h?article=43a6bc08




  Early on, NTSB investigators suspected a faulty fire-warning sensor in the cargo hold. And they tentatively concluded that the cockpit crew had skipped a portion of the pre-flight checklist then failed to restore power to some equipment, making it more difficult to carry out the emergency landing


From http://www.nola.com/traffic/index.ssf/2013/05/united_497_emergency_landing_i.html

Basically it seems there was nothing wrong with the aircraft apart from a faulty smoke sensor. The crew missed this in their preflight checks, noticed it after take-off and then turned off all the electric generators (they were supposed to turn one back on according to the checklist - they didn't). Anyway, they landed the plane OK on emergency power from batteries and RAT.




  with only 28VDC(battery power)*


On the A320 the RAT supplies the ESS bus, not main buses 1 and 2. It powers both the AC ESS bus and the DC ESS bus through the ESS TR (transformer rectifier). So that's not just 28V DC but AC too. 

In the interval after both main generators failing and APU being unavailable, I believe the DC batteries power the DC ESS bus until the RAT is deployed and providing power (which should happen automatically and quickly). So in this short interval there is only DC power on the DC ESS bus.