It is essential that per minute each ${\mathrm{cm}}^2$ of earth receives about 2 calories of heat energy from the sun. This constant is called solar constant S. Express Solar constant in $\mathrm{\_\_\_kW}/{\mathrm{m}}^2$

$\left[\mathrm{Take} 1 \mathrm{cal}=4.2\mathrm{J}\right]$