The average person consumes 2000 kcal a day, which is equal to ~100 W. Furthermore, if one uses the Stefan–Boltzmann law to calculate how much someone loses heat due to radiation, it can be seen that it equals
$$Q=\sigma T^4 \varepsilon A$$
$$Q\approx1000\ W$$
Considering a surface area of ~2 m², an emissivity of 0.98 and a temperature of 36.5 °C.
However, this is clearly much greater than the maximum possible heat output of a human body, and that doesn't even consider convection and conduction, which would make heat loss even greater. So what is wrong with this analysis?
Best Answer
Your calculation of the radiation power emitted by the human body is correct.
But you forgot, that the human also absorbs radiation from the environment. The walls and all the things in your room probably have a temperature around 20 °C, and therefore emit radiation. The radiation power absorbed by the human body is roughly $$Q_\text{absorbed}=\sigma T_\text{environment}^4 \varepsilon A \approx 840 \text{ W}$$
This absorbed power partially compensates for the emitted power. The net radiation power is $$Q_\text{net} = Q_\text{emitted} - Q_\text{absorbed} \approx 1000 \text{ W} - 840 \text{ W} = 160 \text{ W}$$