So if we run 100 billion XP-G's at 30 picowatts a piece, we'd be getting 6.9W of light for 3W of electricity. I think. That many zeros messes with my brain.
Interesting idea. Back-of-the-envelope calculation:
You'd need an area of 1.5 km x 1.5 km and more money than Bill Gates. Not sure you'd even be able to see a dim glow with 6.9W spread over that area. Imagine the cabling you would need and the power losses through that length of cabling!
How about this for an idea: put the die in contact with an efficient photovoltaic solar cell which simultaneously powers the die and extracts energy from it, and you have a machine that extracts heat from the environment and directly produces electricity. I'm sure that violates every law of thermodynamics though.