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When car camping, I use a solar charger to charge my electronics. Sometimes I can place it outside, and other times I will stick it on the dash to charge things.

I can tell that putting it on the dash is less efficient, but I wonder by how much?

  • A further effect is that you have more choice of angle when you don't just drop it on the dash but angle it to catch the sun. Project the image of the panel along the axis of the sunlight and you'll see how much smaller it gets. Any shadows (e.g. from door pillars or rearview mirrors) are also bad. – Chris H Aug 30 '17 at 20:25
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Yes, the solar panel will be a little less efficient on the dashboard than outside because:

  1. Wavelengths absorbed by the windshield. The windshield absorbs most UV, which does still contribute to the solar panel output when present.

  2. Light attenuation due to air/glass interface. The glass will always reflect some light. Think of it this way. You can see the reflection of the sun in the windshield if you look at it from outside. That reflected light is obviously not going to hit the solar panel.

  3. Light attenuation due to windshield angle. When the windshield is directly facing the sun, relatively little of the sunlight is reflected. However, when the sunlight is hitting the windshield at a glancing angle, a significant fraction of the light is reflected and therefore not transmitted to where it hits the solar panel.

  4. Decreased panel efficiency due to temperature. The efficiency of solar panels goes down with increased temperature, becoming 0 at about 150 °C. Usually there will be more air flow around a panel that is by itself outside than inside the car on the dashboard. Also the surrounding dashboard is heated by the sunlight, which warms the air around the panel, which warms the panel more.

With all these effects together, it would not be surprising to see the output of the panel drop by half in some cases.

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Shouldn't be less efficient at all - or at least, approximately as efficient.

Solar panels generate energy utilising the band-gap structure inherent to semiconductor materials. This means that, to generate current, you need to dislodge an electron from the valence band to the conductor band, and the laws of quantum mechanics state that this can only happen if the photon that hits the electron has an energy of at least the band-gap potential. There's also an upper limit, given by the scattering amplitude: if this photon has too much energy, then the probability it'd "hit" an electron is very very small.

In English: solar panels work with light that's in the middle of the spectrum (between UV/gamma and radio/infrared), which includes what we call visible. Windshields are transparent, which means they filter almost no visible light (otherwise they'd be black). In a word: your solar panel behind a windshield glass is as efficient as your eyes behind a windshield glass.

Edit: thanks to ChrisH's comments, we can quantitatively have an idea of this small efficiency loss. Due to optical scattering effects, your sight is probably 5~10% worse behind a windshield, which means your solar panel will also be 5~10% less efficient.

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    Two words: Fresnel reflection. You lose around 5-10% at each air-glass interface – Chris H Aug 30 '17 at 20:18
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    @ChrisH That's a very, very, very good point. I was so focused on the quantum part of it that forgot about the optical. You're totally right: the efficiency lost if due to the fact that part of the incoming wave is scattered and reflected. This shouldn't be so high as 5%, though. Windshields are made to block visible light as little as possible. – QuantumBrick Aug 30 '17 at 20:21
  • See the first graph just above the section heading at wikipedia. Glass has a refractive index of about 1.5. For sunlight you can assume the average of the two polarisations – Chris H Aug 30 '17 at 20:23
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    You have a good point. Just remember windshields are not made purely of glass. They're usually made of polycarbonate layers, acrylic and safety glass. This means that, as opposed to what I said before, their refractive index might be actually higher then 1.5... – QuantumBrick Aug 30 '17 at 20:27
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    I'll definitely add this as a P.S. I'm doing it right now. – QuantumBrick Aug 30 '17 at 20:31
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Yes, a solar charger will less efficient when you put it on the dash rather than outside for a couple of reasons that I can think of offhand in addition to the optical effects already mentioned.

Some of the electricity generation comes from UV photons. Photon for photon, a UV photon has more energy than a visible light photon because it has a higher frequency (photoelectric effect - energy is proportional to frequency). Glass is a very effective UV blocker so you'd be losing part of the spectrum.

Solar cells generally perform less well at higher temperatures. A car dash will often be at a higher temperature (sometimes significantly higher) than the outside temperature, so the performance (conversion efficiency) will be degraded as it gets hotter.

As you've already seen, the efficiency is less when the solar charger is on the dash. You'd probably have to do some comparisons with different conditions to find out how much.

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