Originally Posted by prathmann

What you're ignoring is the data that was provided previously on the total amount of power that's available in the incoming sunlight. This is a little under 1 W/sq.in. So even if the solar cells were 100% efficient and aimed perfectly perpendicular to the incoming light, this is the most power you could ever hope to get out of them.

Currently even the best solar cells are way under 100% efficiency - and the only way that you gain anything by stacking them is if they remain very inefficient. That is, if you have one panel that only extracts energy from the highest energy photons (violet to near UV) and lets the rest of the light through, then that panel is rather inefficient but the remaining energy in the lower energy photons can still be extracted by panels lower in the stack with each panel designed for a particular range of photon energy. But if you ever achieved a single solar panel with high efficiency then it would make no sense to stack them since the top panel would already have extracted all the available energy and the lower panels would not be adding any value.

So the most power you could ever hope to get out of a 12" x 12" panel is about 120 W assuming perfect efficiency, a clear sky, sun high overhead, etc. Making a stack of panels doesn't let you get any more power than this out of a square foot of collector, but it might be a practical way to improve the efficiency that we can currently get from a single panel.