How much power is needed by aircraft IFE systems during flight?
I don't have specifics but lets extrapolate for a minute (using the Boeing 777-300 for size filled with lets say 500 seats). Assuming you are talking about IFE systems where there is a screen in every seat then you will most likely have screens similar to this. While not exactly what the airplane uses I would think they are similar. The specs for that screen claim it takes &lt;= 8W. Lets round it out and say it takes 8w. Looking at our 500 seats thats 4000W of power. I can only assume there is a computer or two of sorts running those displays, lets say they have 500W (typical desktop supply on the more powerful side) and there are 2 of them. Thats another 1000W so you are at 5000W there. Assuming you have 120V power P=IV thats about 41 Amps of current you will draw from the system. I am running this assumption that all the screens are on at all times as I assume the system is built for that. 

Cabin lighting is tough as I have no idea how many bulbs they have in there. I know that each seat has its own little lamp so you are looking at 500 little lamps running in the 1W-10W range. So maybe another 2000W for the individual lights. Overhead is tough since you cant really see the individual bulbs. It should also be noted that LED's are becoming more wide spread and draw way less power than traditional incandescent bulbs which will greatly reduce the amount of power drawn by the interior lights.