How much power is used by cabin lighting systems during flight when all the lights are ON?
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 & 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.
I cannot find Mike Foxtrot's answer so this might be duplication. The power consumed is the power used by each lamp multiplied by the number of lamps. Every type of aircraft is likely to have its own type of lamp & its own number of lamps. Also, cabin lamps can be dimmed & reading lamps are a variable load. LEDs are getting more powerful (light output/energy consumed) so fewer lamps are needed, but this comes at the price of more heat generated; this requires heatsinks. So the designer will select lamps on the basis of light output, area to be illuminated, power consumption, heat output & heatsink size. So a definitive answer to your question could only apply to one type of aircraft.
According to this link, an Airbus A-321 has around 100 LED units for cabin lighting which have 45 individual diodes each. Do you have 4500 diodes in the cabin in a single aisle plane, which means at 0.05W per LED estimation, the cabin lighting would require 225 W of power.
p.s. This does not include reading lamps (most probably) and only the aisle lighting.