buy a 250w bulb. i don't use one now, but i did when the bitch had pups, imo there is no need for one, draft free kennel, dry bedding, up their food intake a bit and they will be fine, i have 4 dogs that all sleep together in a 8ftx4ft kennel, it has a raised floor of 4 inches with polystyrene insulation and 3 inches insulation on all sides, i have also lowered the ceiling from 4ft to 2ft which can be removed in summer.