Efficiency of solar panels ? transition line

I want to calculate the efficiency of solar panel, I know that the answer need to be approximate 15% because this information I find in the website of one company that manufacture the solar panels, but I don?t know how they calculate this?
Maybe it can be correlated to transition line?
I think it, but I am not sure, because maybe it is related to the light sun that touch the atmosphere and part of this return back to the space, part of this absorb in the atmosphere and only part of this pass to the earth and after this it arrive to the clouds and again part of the sun light return, absorb and pass, I search more information about that and how to calculate this.
If the solar panels will be in the space outside of the atmosphere so it will not have the problem of part of the light return or absorb but it will be other problem that is how to pass the energy from light that is outside the atmosphere to electrical energy that is in the earth.
Do you have any idea how to do it with minimum of lost of the energy?
How it possible to grow up the efficiency of the solar panels?
If you have any information, idea, links that are correlated to this issue so I will glad if you can write it in this thread.