$${\begin{align*}P_\text{res} &\approx \mathrm{(5,67 \times 10^{-8}\, W/m^2 \cdot K^4)(0,80)\times \\ \times(5,03 \times 10^{-3}\, m^2)[(320\,K)^4 - (308\, K)^4]} \\&= 0,34\, \mathrm{W} \end{align*}}$$