It's pretty much dependent on the size of the enclosure and how much heat is allowed to circulate and dissipate. Take the 20L U.S. gallon and the 29 U.S. Gallon tanks.
They are both 30'' x 12", however the 29 is 6 inches taller and by putting a light fixture with a 60 watt ceramic heater on the top of a screen top it may only keep the temperature at 80 degrees F, whereas when the identical fixture placed atop the 20L will provide temps closer to 90 degrees F.
What exactly were you looking for and what size enclosure are you looking to heat and what is your target-temperature?
Depends on many factors. As victor creed has said the size of the enclosure and also the temperature of the room. If your room is warm the ceramic won't need to work as hard to raise the temps whereas in a very cold room the ceramic will be on a lot.