I agree with this. I have tried several setups. The Anker 21w with shaded large capacity batteries is my favorite and most effective. It has also lasted a long time for me.
PD power banks on solar? Other non-PD sources?
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You can charge them, I’ve charged a 16000 powerbank with a 24watt solar panel (competitor sent me one to test) and it took all day to charge. But it was free power and perfect for off grid days.
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You need to be patient using these slow ways of charging.
But as you said:
If there is no electricity at all you need patience anyway. 
Same is with our thermal solar systems (hot water) on the roof:
In summer we have too much hot water, in winter when you would really need it to support the heating system, there is NO !
(At the moment a pile of snow at the collectors)
Solution would be to bury a huge, well isolated tank deep in the earth and to store the hot water in summer for wintertime.
But its quite a good feeling.
I had really remorse in summer when the old
system started to boil water wasting fuel oil.
Now we use a gas heating & solar system (less pollution, no more bad smell in the basement) 
New built houses are equipped with heating pumps and floor heating system and are isolated perfectly.
But our old house is a good, solid house. 
Well, I have a handheld vacuum cleaner with rechargeable batteries. And a small solar panel. So I mounted the hook for the cleaner next to the window, put the panel in the low window corner, connected the panel to he hook - and so I always have a charged cleaner.
Is on duty for many years now and still working perfectly.
So you could do something similar with your powerbank. Look for a nice, sunny window …
And if you really have good sun, you even can think of a charging station for the Robo Vac - connected to a solar panel and not to the mains …
Depending on where they live that may not be feasible. A lot of newer windows have a 7v coating so to prevent us damage to your furniture. This would block the required uv Ray’s for the solar panel to work too, making it an expensive paperweight unless taken outdoors
will still work but charge slower from the highest frequency let through e.g. blue light.
Worth pointing out that anything which can be reasonably recharged from a solar panel will likely cost in electricity over many years less cost of the solar panel. When that breaks down is if you’re looking at laying down a long cable. I can do the math if anyone challenges me.
From a “green” perspective go large or don’t bother. From a camping off-grid perspective need to go portable, large as you can carry and be away from wall sockets roughly by about a week or more to be worthwhile.
I blame bad schools, they let people go into the world not able to do math. 
@furkist the window does dramatically impact performance, interestingly. It’s the difference between 10w and 3w. I actually metered it - check that out here:
I agree window should and does reduce but I’m surprised by a factor of 3. I know hotter solar panels are less efficient so warmer indoors, against a warmer window or a warmer room?
Over 10 years ago the common use case was a seldom used motor bike or spare vehicle and the garage windows were just to keep the rain out but not that intentionally UV blocking and you’d have solar there doing a trickle charge to keep battery topped up for months at a time. You’re showing in effect trickle charge indoors. Now with modern electronics ubiquitous that garage which didn’t electricity probably now does so the use case has reduced.
BTW high latitude can offset high latitude as less air to absorb UV, and snow reflects UV so a Montana winter outdoor may make as much solar per hour as a Montana summer outdoor (just for fewer hours). Are you measuring all of these through the seasons?
Down here in California near a major fault line we are encouraged to keep a pack to last 3+ days and inside of mine is a Powercore and my 21W Anker Solar.
I did not know this. It is really surprising me. In the actual case with the cleaner, the panel first was built in a plastic box with - of course - plastic cover. The cover was not “glass” clear, so i removed the whole box. Therefore the panel is not more suitable for outside use.
But be sure: I will keep your remark in mind, in case of any future application. Thanks a lot , @ryandhazen !!!
I tried my best to control for the factors between indoor and outdoor, including using a compass to measure the orientation of the panel to the sun’s location. The other factors temperature, reflection, etc) I would consider an existential difference between the indoor and outdoor applications - basically, I wanted to know if I could get as much charge from setting up my panel in my office vs setting it up outside. That room (my office) certainly was warmer than the outdoors that day (by about 50 degrees F I’d say), so it could certainly be a factor. However, I’ve measured those panels performing at 10w on one outlet (into that same powercore II) or better in summer months when the ambient temp was much hotter than the office (by about another 25 degrees F), so I wouldn’t think the temp could have such a great impact.
You point out another good factor to keep in mind, that the quality of the window is important. The window I have in my office is part of a college library (not a residence) that was built to meet stricter standards than the average home. It may be that those industrial windows have additional coatings or glass types that aren’t your average window. They were built to store books, after all, and may have some additional protection so that the books aren’t slowly degraded by sunlight on their shelves. They may also be stronger to add R value on such a large, open building as this library, saving the institution money in the process. I just don’t know any of that for sure. It recommends a test at home (both in my living room and garage). I think i’ll get that new meter and get to work.
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Because of this discussion I doublechecked the window where I mounted the panel:
It is glass, 3 (three) levels, but not coated. Now we have a lot of snowfall and no sun.
But I will keep this in mind and check, when the weather is better - this may need some time …
I have a small, basic meter built into the holder (V and A, swithable) to “control” the operating status. Not really precise, but enough to show the difference, when the position of the panel is changed.
So long as you’re thinking trickle charge you’ll get some benefit. So the solar cells reduce their voltage when behind glass, the DC-DC converter in the electronics have to reduce the current to get the voltage up nearer to 5V so you’ll observe the voltage drop a little but the current drop significantly.
The voltage out of cells is the delta voltage left over after electrons come out of their potential well so in some situations if the cells see too low a light frequency they will generate no energy whatsoever. Unlikely as white glass will let blue light in.
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Well, since the whole apparatus - solar cell and cleaner - is working fine now for quite long time, (several years) , I do not change it. Maybe I will replace the ni mh batteries, when they show reduced power.
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Is yours a portable system or one that powers your home? Mine is just for small devices. I can power my phone, ipad, flashlights, and laptop on solar for most of the Summer, but I don’t have anything to power my home yet.
The panel I use is also small: 12 x 15 cm. It fits nicely in a corner of the window, it does not disturb the view. So I use it just for the cleaner. Could be good also for phone. But since it is switched off most of the time, it needs very few charging …
But you give me an idea: I will make a connector between panel and cleaner, so I can connect other things - flashlight …
For the home, you need big panels and also some electronics for charge and so on. But you sure do know this. As I live in an appartment, I have no roof where to put the panels.
I also have the same setup and always wondered how much charge I really get as I have not tested to a great extent. If there is enough solar charging and time passes will I get a complete charge. I have this setup for camping purposes and charging solar lighing connected by USB.
I did some metering on this panel, check out that post here:
I also made a video review, though the iPhone actually charges faster than I metered - see the in the discussion of the post below:
@furkist it might help to know that I found out that the window reduces the amount of charge you get from one of these small panels.
I know how complicated those big panels are. I have a few friends with with those setups (one with used car batteries even) and they take a lot of work to maintain!
Well, here is a picture of the complete setup:
On the left you see the Vac, on the window desk the robos. So it is my version of RoboVac. Above of the Robo you see the back side of the solar panel - outside is night at the moment.
On the “holder” you see my measuring equipment: I used a level meter from the scratch box (do you say so ?) added a few resistors and a switch. So I can “measure” Voltage and Amps - very rough but enough for me. The scala of the meter I made with pencil and colour pen …
So you see - I am a technical freak combined with model builder combined with …you name it.
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This is so cool. And yes “scratch” works, “scrap” may be more common. Either way its very clever. I think you could certainly power a phone other small items with this setup if you added power bank on to it. Also i wonder if you placed the panel outside the window (attached to the outside wall somehow, or hanging from the window ledge) if you would measure more power? My Anker solar panel definitely gets more power when it is in direct sun outside than when inside looking through a window.
Cool robot by the way!
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The panel is inside. As I mentioned: it was in a plastic box with “transparent” cover. But you could not see the panels , and the cover became “yellow”. So I removed the complete box. And so the panels have no cover (as you can see on the cable “fixing” - this is original …) - no protection Therefore I have them inside.
The solution would be: build a new box - but then this box has to be covered with some transparent cover. I made for my Vespa scooter a new “glass” for the speedo - the old was plastic and was damaged - the new I built of acryl - and after one year I could not read the speedo .- the acryl was milky.
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