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The solar panels that can be bought today typically respond to a relatively small portion of the emission spectrum of the sun. Not so coincidentally, solar panels are rather inefficient, with typical output on the order of a few hundred watts per meter squared. Which inspires the question, what if we could capture 100% of the energy from along the entire electromagnetic spectrum?

Clearly, the second law of thermodynamics forbids perfect efficiency just as it forbids perpetual motion machines, but how much solar energy is there along the entire spectrum? What about from space, where our atmosphere doesn't modulate the energy passing through?

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how much solar energy is there along the entire spectrum

It depends on the conditions under which it's measured, and - given that the sun's output varies with time - when it's measured.

Standard test conditions are for an air-mass index of 1.5, and under those conditions, the figure is $1000 Wm^{-2}$

The standard figure for the edge of space is $1353 \pm 21 Wm^{-2}$

100% efficiency isn't possible for PV, and isn't relevant either. Typically the only thing that matters is cost per unit electricity delivered. Except when it's PV for space applications, when it's power per unit mass that matters. But efficiency? The only time it matters, is if you've got a tiny space, need to harvest lots of solar power, and cost is not an issue. These cases are so rare that there's effectively no interest.

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