Solar energy produced is a function of how many photons from the sun strike your solar cell, and how many of these photons actually get an electron moving to create a current. The number of photons, or amount of sunshine, that strikes your solar cell needs to be determined to see how many cells or solar panels you will need for your solar system. This energy will vary considerably depending on your latitude, the time of year, and cloud cover that your region can expect.
The two components to incident solar energy that most flat-plate and evacuated tube solar panels can use are: Direct radiation and diffuse radiation. Understand that concentrating panels are not able to benefit from diffuse radiation.
First, direct radiation means that incident energy comes as though it was a beam of straight rays coming from a source. A good example is, clear, dry days, with no dust in the clouds. That is direct, meaning it comes straight from the sun.
On cloudy cover days, with rain, humidity, air pollution, or dust in the atmosphere, much of the solar radiation is all scattered, meaning the energy that comes from the sun go in different directions. We call this diffuse radiation, it’s still available solar energy but its rate is smaller. On days like these a rooftop of a home can receive an average of 1000 Btu’s per sq. ft. for that day. On a good day of direct radiation it could double or triple that. It all depends on the location and time of year, diffuse radiation still produces anywhere from 1/4 to 1/2 of what direct radiation will produce. However some devices like a Solar trickle charger for a car battery will continue to work effectively as they do not require much sunlight.