Gotcha. Yep, one per side is sufficient. I have my enclosure in a room that stays around 68F. I have my 40w RHP on a herpstat II, and aside from the first time I plugged it in, I've never seen it running over 70%. I have mine slightly off-center on each side and that creates a gradient of about 8-10 degrees. I also have heat tape, but it rarely ever comes on at all.