We're extracting almost as much power from basic solar panels as physics allows, but it hasn't slowed down the global solar market much.

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There are plenty of ways to capture solar power, but the cheapest, most common tool by far is the crystalline silicon solar panel. Researchers in Japan have created the most efficient version yet — it can convert more than 26 percent of sunlight to usable electricity.

It doesn't sound like much, but it's getting close to the physical limits of efficiency for these kinds of basic panels. The behavior of photons puts a hard cap on the amount of power we can squeeze out of them.

We can get around this limit by stacking more semiconductors in the cells, or using concentrators that focus more sunlight in less space, which gets us closer to 86 percent efficiency.

But these high-tech options are nowhere near as common or as cheap as basic silicon panels, especially in developing markets. Costs have plummeted, and investment has rocketed. Under the right conditions, solar power from silicon panels is now cheaper than fossil fuels.

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So there's not much incentive to replace all of that progress. It's almost always more efficient to keep doing what you're doing than to change a whole industry over to a new technology at once.

But energy firms are optimistic that tech is going to keep improving. Silicon cells have gotten drastically cheaper in the last decades. They think concentrators eventually will, too.