Welcome to Physics Forums! You are entering into an area of science where the use of exact terms is absolutely necessary. You need to learn basic definitions in this field so that you can understand the processes and then communicate the scientific facts to others accurately.
To calculate the amount of sunlight reaching the ground, both the elliptical orbit of the Earth and the attenuation by the Earth's atmosphere have to be taken into account.
http://en.wikipedia.org/wiki/Sunlight
For your project of dehydrating, however, it may not be necessary to do this calculation.
"The term "radiant energy" is most commonly used in the fields of radiometry, solar energy, heating and lighting, Radiant energy is one of the mechanisms by which energy can enter or leave an open system. Such a system can be man-made, such as a solar energy collector, or natural, such as the Earth's atmosphere."
http://en.wikipedia.org/wiki/Radiant_energy
One of the favorite sayings among engineers is "Don't spend time re-inventing the wheel." The idea applies to your project: search out what others have already done and use the best ideas suitable to your project. For example:
"Solar Food Dehydrator
The construction of a solar food dehydrator is not tremendously different than that of a solar box oven, with a wooden frame holding walls and a top made of glass for sunlight to get through to the food inside. The food to be dehydrated sits on trays made of wire screens, the tiny openings of which allow proper air flow to and through the food.
A dehydrator requires the presence of small, interior solar-powered fans to help the drying/aerating process and to keep the interior from becoming so hot that the food accidentally cooks."
http://www.ehow.com/list_6753580_solar-collector-projects.html
Google search terms that bring lots more suggestions: “sunlight radiant energy food dehydrator”
