How is the ALSF-II approach lighting system built and powered?
The specifications for an ALSF-2 (and other approach lighting systems) can be found in FAA JO 6850.2B - Conveniently titled "Visual Guidance Lighting Systems."
As an FAA order these are obviously US-specific, but other jurisdictions' guidance is probably similar.

The ALSF-2 is a High Intensity lighting system, so lights used in the system are defined in the FAA order as follows:


  High Intensity Systems.
  
  
  Specification FAA-E-2408b, 300-watt, 20A, PAR-56 lamps for steady-burning, aboveground, white lights with Specification FAA-E-982 lampholder.
  Specification FAA-E-2408b, 500-watt, 20A, PAR-56 lamps for steady-burning, aboveground, colored lights with Specification FAA-E-982 lampholder.
  Specifications FAA-E-2952 or FAA-E-2491 approach lights, 500-watt, for all semiflush lights.
  Specifications FAA-E-2628b or FAA-E-2689a for all sequenced flashers.
  


From the number of required lights we can then derive the total system power used for an FAA-Specified ALSF-2 lighting system -- As I'm not fond of doing math it's fortunate that some manufacturers have already done this for us, and a full system (with all the flashers) requires approximately 104kW (varying slightly depending on input voltage). Actual utility power consumption will vary based on the efficiency of certain components but will likely shake out to something around 104kVA since the incandescent lamps have a power factor close to 1.0.