how to convert/calculate \$\text{kJ}/\text{m}^2\$ to \$\text{W}/\text{m}^2\$
If your irradiance data is per hour, then these are the units:
$$\frac{kJ}{hm^2}$$
A joule is a watt-second, k is 1000, there are 3600 seconds in an hour:
$$=\frac{1000Ws}{m^2h(3600s/h)}$$
Cancelling units:
$$\frac{kJ}{hm^2}=\frac{0.2777W}{m^2}$$
Or
$$\frac{3.6kJ}{hm^2}=\frac{W}{m^2}$$