How much power is used by cabin lighting systems during flight?
Almost all lighting systems installed today use Light Emitting Diodes for weight savings, reduced power use and less maintenance. Compared to other aircraft systems, their power use is pretty insignificant. For the reading lights, I found the following quote:


  Passenger reading lights in halogen technology use 11 watts, while the
  latest LED products use only one watt. Source 


The actual ceiling ceiling lighting power consumption seems more difficult to find. SAAB in a brochure for converting fluorescent to LED lights for their SAAB 340 &amp; 2000 models states:


  Power consumption is reduced from 8 watts per 12 inch tube to just 5,1
  watts.


Emteq suggests savings of around 15% for LED compared to fluorescent lights, so SAAB's suggestion would be a bit more. This manufacturer quotes 10W for a 16-inch (40cm) LED strip running off the 28V aircraft DC system. Needless to say, power consumption depends on the wattage of light installed.