Actually it depends on the ambient average temp. If the room is say zero you will need a lot of wattage or BTU's to heat the cage to 74-76. If the ambient is say 70 F then between 100-200 watts might work.
well I ended up getting rid of that cage i built, he loved it, used the ramp and tubes all the time, but it wasn't warm enough and slowly he would sleep more and more and become less active. but now he seems to be more active with this smaller cage and seems to run on the wheel more from the poop content. so this can be closed. i have a 150w ceramic heater for a 105 liter bin. i don't have a thermometer yet but i'm sure it's fine