The sun radiates electromagnetic energy at the rate of $3.9 \times 10^{26}\,W$. It's radius is $6.96 \times 10^8\,m$. The intensity of sun light at the solar surface will be (in $W/m^2$)
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intensity $=\frac{\text { energy radiated }}{\text { area }}=\frac{\mathrm{E}}{4 \pi \mathrm{R}^{2}}$
$=\frac{3.9 \times 10^{26}}{4 \times \frac{22}{7} \times\left(7 \times 10^{8}\right)^{2}}$
$=\frac{10^{10}}{22 \times 7}=\frac{1000}{54} \times 10^{7} \simeq 6.4 \times 10^{7} \mathrm{W} / \mathrm{m}^{2}$
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