Considering average human body area is around 2 square meters and body temperature is around 37 degrees Celsius I get around 1000 W of radiation from the human body according to this calculator: http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/radfrac.h...
Or to put that in terms of net heat loss, in the 0-30C range you're looking at about 10 watts for every degree of difference between your skin and the environment.
Human skin temperature is considerably lower. (Or to think of it from another perspective, the system which is insulating your core temperature includes layers inside your body – skin, etc. – in addition to clothing, blanket, etc.)
Backing out human power consumption from caloric requirements comes closer to 120W constant output, at 2,500 kcal/day.
The range is a low of about 50--75W (1,000--1,500 kcal, effectively starvation levels), and a maximum ceiling of around 485W (10,000 kcal day, perhaps a elite endurance athlete). Well below 1 kW in all cases.
