system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are solar panels only like ~20% efficient (i know there's higher and lower, but why are they so inefficient, why can't they be 90% efficient for example) ?

I was looking into getting solar panels and a battery set up and its costs, and noticed that efficiency at 20% is considered high, what prevents them from being high efficiency, in the 80% or 90% range?

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EDIT: Thank you guys so much for your answers! This is incredibly interesting!
gpt
Physics and cost.

The theoretical efficiency limit is 95%. This is solely determined by the temperature of the sun and the temperature of earth. Whatever you do, a higher efficiency is never possible.

However, there are a couple of limitations. First, the solar panel has to *send out* light as well: the temperature of the panel is above absolute zero, so it emits heat. This brings it down to 86.8%. But that assumes that the incoming light comes from every direction at once. In practice, the sun only covers a small part of the sky, bringing it even further down to 68.7%. And that's still with a perfect solar cell! That assumes the cell is infinitely thick and has zero losses.

If we try to actually build cells, the best we can currently do is around 44.4%, which isn't too bad! But those cells consist of multiple layers, use exotic materials, and are very expensive to construct. It is *way* cheaper to construct less complicated cells. Turns out we don't really care about the absolute efficiency: there is plenty of sunlight available. We just want the most power at the lowest cost.

The most common (and cheapest) cell type is "single-junction". The theoretical efficiency limit for those is 33.16%. Then we have some losses due to the protective coating, the wiring, being unable to cover 100% of the panel with cells, and loooots of other small stuff.

So yeah, it might not sound like much, but an efficiency in the 20ish% isn't too bad. Don't expect anything over 30% soon, because we're already rapidly approaching the limits of physics!