I don't remember what number exactly but it's in the single digits. This number represents how efficient solar panels are at capturing the sun's rays and converting it into electricity.Why?
There are a lot of solar panels available today's. The problem of the efficiency is the proccess of converting, these cells use the photons of the sun to react with the ( Si compound inside the cell) due to the little thing that cell's are only a fraction of these energy is converted, remember that solar cells are paper thin and the really problem of these cells are the voltage not the amps. A single 3x6 inches solar cells can produce 3.6 amps but only 0.5 volts.
The most important thing is for how much time the sun shines at your place. If you are living at a place where the sun shines just for to 2 hours then installing a solar panel will be of no use. But if sun shines at least 9-0 hours then solar panels will be in use.
The latter link has a decent explanation, if not a bit technical: One of the most fundamental limitations on solar cell efficiency is the band gap of the semiconductor from which the cell is made. In a photovoltaic cell, negatively doped (n-type) material, with extra electrons in its otherwise empty conduction band, makes a junction with positively doped (p-type) material, with extra holes in the band otherwise filled with valence electrons.
I don't remember what number exactly but it's in the single digits. This number represents how efficient solar panels are at capturing the sun's rays and converting it into electricity