What is the value of the solar downward directed radiation flux reaching the surface in your town at noon on 4th July. 20% coverage of cumulus cloud.
The irradiance on a surface located on Earth and pointed perpendicular to the Sun is called the Solar Constant. Irradiance has unit $\displaystyle \text{W/m}^2$. 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:
$\displaystyle I=\cos(\theta)\cdot S$
in which $\displaystyle \theta$ is the angle between the surface perpendicular and the Sun and $\displaystyle S$ 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.