Solar downward directed radiation flux

The irradiance on a surface located on Earth and pointed perpendicular to the Sun is called the Solar Constant. Irradiance has unit $\text{W/m}^2$ . If you look up Solar Constant on Wikipedia you'll quickly find its value.

In your case your surface is probably not pointed perpendicular to the Sun. The effective irradiance then becomes:
$I=\cos(\theta)\cdot S$
in which $\theta$ is the angle between the surface perpendicular and the Sun and $S$ is the Solar Constant. You can easily find the value of the angle by using this nifty calculator.

Then still multiply by the loss factor due to cloud coverage.