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?
Yes, the solar panel will be a little less efficient on the dashboard than outside because:
With all these effects together, it would not be surprising to see the output of the panel drop by half in some cases.
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.
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.