You should start a new thread for this... this is a several month old thread about a poll on the best heaters, so this is rather off topic here unless your giving your opinion on your favorite heater. But I dont know if your question could even be answered. There are so many variables, like the size and shape of the tank, surface area, wether or not your base is insulated (or back for that matter), thickness and type of aquarium walls, canopy, lighting, filtration,etc. etc. Perhaps there is some rule of thumb I dont know, or perhaps some have actually measured it??? I havent heard of any such things though.