90 watt maybe to much, we have a homemade one that measures 24''x12''x12'' we started using a 25 watt bulb and our temps went well over 100 I think it stabilized at 106f we then switched our bulb to a 15 watt and the temp stays stable at between 99-100. We have about 12 dime size hole around the top of the bator. This allows for some air flow. As of now 6 of them are covered and we are holding steady at 100. If we see the temp falling we cover a hole, and if its raising we uncover one. It works pretty good as a back up bator. Right now we have a LG and our homemade incubator... In the LG we have marans in lock down so in our homemade one we have 5 mystery eggs going on day 14... After our marans hatch and we clean the LG our mystery eggs will go into lock down in the LG bator...