I use one per 15x15 room. having 3 rooms with snakes in them heated to 81F and 51% room humidity is fine wiht me. Like i said i run 3 of these on top of the snakes heating and my lights in house and i barely break 150$ a month in electric.

Im not gonna argue over it. do what you want but the OP was looking for a cheap way to keep room temps up not go all out of something thats over kill.