How is the 840 solar charging supposed to work?

Having done a bit reading around I can probably answer some of this myself...

DC rainmaker's review concludes the solar option is generally not worth it and you'd better off spending the money on ice cream. He observes (as I have)  that whilst there are small gains whilst riding, charging whilst powered off seems ineffective. The battery life is good so most of the time you're not going to need solar charging even it it worked.

It's not going to do much for you are on a multiday tour away from a power source. This also appears to be the case for its older and bigger brother the 1040. 

So the solar option appears to be fairly useless - almost verging on mis-selling. I get the impression that this is a design fault - either hardware or software. From a electronic engineer's point of view this looks resolvable and was intended to be useful. Can it be fixed?

It would be useful to get a response from Garmin.

My Instinct 2 solar watch performs rather more convincingly in this respect 

According to Garmin, it's supposed to charge even when powered off.

https://support.garmin.com/en-US/?faq=6pHHY9vjCs7MG9UZjJLFh5

Does Solar Gain Take Place While the Edge is Off?

Yes, solar gain can take place while the Edge is powered off or asleep.

My instinct Solar (first gen) and the 840 Solar are expensive because of the solar feature, and I notice no benefits to having solar charging. I can be out all day in the sun, for 12h+ and while the watch and edge report that they gained some power, I feel like they die much faster soon after. Like it was faking being actually charged or that the charge was meaningless.

Very disappointing. 

On a 3 hour ride this week I got 24 minutes of charge which equates to about 1% according the Garmin information. This is roughly (or even slightly better) what is expected given it is September and the unit would not have been getting 100% solar gain. Is the 24 minute calculation based the current settings such as satellite selection?  I run with backlight off, 50% time viewing map and Multi- GNSS.

What I have yet to see is any visible gain when sleeping or powered off.  In principle I'd expect to see 2-3% after 5/6 hours in the sun. I'll try a longer test - sun availability permitting.

If the Garmin figures are valid then to get an 1 hours worth of charge (demanding use) requires 4.5 hours in the sun. To get, say, 8 hours worth will take 36 hours of charging.

Even if it does work as Garmin says - this is a marginal benefit IMO.  

I guess we can take as a guide the fact that Garmin has NOT extended the “solar” options to the 1050/850/550 , perhaps due to it not really working all that well and certainly not worth the extra cost and slight negative impact on the screen brightness? When it was first released on the 1040 several “ultra” cycling riders commented favourably on the solar option for them but translated to the smaller x40 size does not seem to work well (enough), good idea but?

these new colored LCD panels (and the rest parts) need more energy than the solar panel can produce , so meaningless to put in it, with the dead of the MIPS i think this solar thing is dead, a denser battery is better choice (but that is not 0.05dollar as the actual one... :) )

Well, well now that the 850/550 were realeased, battery LIFE has a much different meaning :D I hoped for a balanced 850, but now I will buy the solar 840, have awesome battery life and save some money as well.

A few calculations underline the very marginal benefit of solar...

Taking an example of 3, 8 hour rides (24 hours) in full sun. The Garmin figures suggest 16% charge or 4 hours of extra use. The figure could be less in practice due to shade etc.

As the battery life (no solar) is likely to be 26 – 42 hours I can’t really see a usage case where the solar is useful in practice. A best you’ll need to USB charge slightly less frequently. Better batteries have made solar a bit of a gimmick IMO.  

If you were riding off grid for multiple days solar charging isn’t going to contribute much. You’ll probably be carrying a battery pack / solar charger and use it for other kit in this scenario.

A few more calculations underline the very marginal benefit of solar...

Taking an example of 3, 8 hour rides (24 hours) in full sun. The Garmin figures suggest 16% charge or 4 hours of extra use. The figure could be less in practice due to shade etc.

As the battery life (no solar) is likely to be 26 – 42 hours I can’t really see a usage case where the solar is useful in practice. A best you’ll need to USB charge slightly less frequently. Better batteries have made solar a bit of a gimmick IMO.  

If you were riding off grid for multiple days solar charging isn’t going to contribute much. You’ll probably be carrying a battery pack / solar charger and use it for other kit in this scenario.

Yeah, 4 hours gain in 24 hours is about the maximum I've seen. I live in Northern Scotland so it's very rare that I even get that ratio. Normal Summer riding might get me 3 in 24 and  it's usually less. In Winter it's basically zero.

If I'd known this when I bought the device I'd have bought the non-Solar version which, I believe, has a slightly brighter display.