The pillow top cover accommodates a removable heater on a sturdy pet cot frame. Easy to assemble, no tools required. The small has a 4 watt heater and the medium has a 13 watt heater which can be removed and stored in the off-season. Polyester cover is machine washable. Super comfy heated pet cot pad Great for small dogs or cats Removable heater for summertime use Assembles without tools Rubber non-skid feet