Earth gets its radiation input primarily from Sun, then from reflected sunlight from Moon, and stars. Among these, what is the share of stars (or sources outside solar system) to this total radiation input of Earth. Is it negligible compared to solar radiation? (I am referring to top-of-atmosphere situation.)
It is negligible.
The amount of energy per square metre per second is called the "flux". The flux of the brightest star in the sky, Sirius, is one ten billionth of the sun. That mean the energy the earth receives from the sun in one second is the same as the energy that it gets from Sirius in about 300 years. And Sirius (at magnitude -1.47) is by far the brightest star. All the others add next to nothing.
Posted as an answer mainly so it won't vanish as a comment. Anyone is welcome to take this and refine it in their own answer.
One lux = one lumen per square meter.
Bright moonlight is about 0.5 lux.
Full noon sunlight about 100,000 lux.
Stumble about in the dark and see dim shapes is 0.1 lux or less.
Brightest of starlights is going to be well under bright moonlight - maybe 0.01 - 0.1 lux range. At a very very very rough calculation that puts sunlight at about 10E6 to 10E7 above starlight. Adjust for asymmetries across the earth's lit disk and much else and that is liable to still give a result somewhere over a million to one.
30 seconds f/3.5 ISO 3200. Brightness value -6.3 (dark!)