We have a really long homemade incubator and use two bulbs, a 40w and a 60w, one on each end. We liked the idea of heat coming from each end. Two 40w's were too little and two 60w's were too hot. We regulate our incubator temp by a series of holes in the lid over the light bulbs. If it is getting too hot we open a few holes (usually over the 60w end) and if it is too cold they are closed up.