The irradiance on a surface located on Earth and pointed perpendicular to the Sun is called the Solar Constant. Irradiance has unit . 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:
in which is the angle between the surface perpendicular and the Sun and 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.