Outside on the top of the Nellis array are several broken solar panels. You will need to pass a Repair check of 65, or scavenged parts from HELIOS One which require 20 Repair skill to scavenge, in order to repair these solar panels. No reputation with the Boomers is gained directly for fixing the solar panels; however, completing the quest Sunshine Boogie will earn you reputation with that faction.
There is a computer terminal by the gate. Until the generators are powered back on, it displays the error message "ERROR #P38C4K" "Generator power source unavailable" It checks array efficiency when the array is operational again (120% at best.)
The inside of the facility is quite simple. Inside the main entrance is a small room with a doorway straight ahead and another door to the left. The forward door leads to an enclosed gangway that goes over the main generator room below. To the left is another room that has a door to the right. This door leads to an exposed gangway with stairs that lead down to the lower level. Below the building is sectioned into two areas. From the stairs you are in the smaller of the two sections with electric transformers and generators. The larger room contains more electrical equipment, an exit, shelving with ammunition containers, artillery shells, and your run of the mill ant garbage. The exit leads to another exit to the outside by way of a tunnel.
The real-life Nellis Air Force Base does have a solar power plant, constructed in 2007.
It is interesting to note that the parts to repair the solar array have been scavenged from solar reflectors, i.e. mirrors, while the Nellis array consists of solar panels, i.e. photovoltaic cells. The two use an entirely different technology to harvest solar energy. It is possible that Helios One is a photovoltaic thermal hybrid solar collector, which uses both types of technology simultaneously for greater efficiency. This would allow the parts to be compatible with both arrays.
It seems that once the quest Volare! is completed, there is no access to the interior of the array without a key of some sort.