A difference of many orders of magnitude that you can't compensate by the size of solar panel on a small device.
The latest version device in question (the OLPC) is ~5W under normal usage and less than 1W when idle.
Im not sure about an integrated solar panel - im not sure i would want to keep my laptop exposed to the sun - but i was able to keep a 2015 macbook charged with a small solar panel while travelling (approx 4 hours a day of heavy usage).
Obv depends on time of year, location, weather, etc.
There are some electronic price tag products, wireless keyboards, decorative keychain tags, etc. that could work on solar only, but even those are rare.
e: Forgot digital wristwatches. They seem to work fine.
You could probably power such a calculator from ambient radiowaves, using a galena crystal...
I really like the Logitech K700 solar keyboards, with two caveats:
- In a couple of mine, the included battery seems to have lost the ability to keep a charge well before you would expect (i.e. a year). This is a bit of an annoyance in that it is a rechargable button cell, not your run of the mill AA/AAA sized NIMH/Lithium bit.
- The build quality on these as far as sturdiness/etc isn't great. And if it ever gets 'flexed' too much, you wind up with broken traces internally, leading to issues where the juice doesn't flow one way or another unless you flex it back in the right way.
I love them in the office, but at home they just don't handle the use of clumsy roommates.
Because they are ultra-low powered devices. If tiny solar cells can power that kind of device, then it can also be powered through kinetic energy (i.e. user's movement) as well ... Or you could spend $5 and put a standard watch-type battery in there which will last you years. So even there solar panels are a bit gimmicky. But ok, do whatever you like.
At least in my area, the speed warning signs tend to get moved to a different location a couple of times a year, and not having to run any wiring probably makes that significantly easier.
The solar charging is probably the biggest feature I'm missing in my smartwatch, but at least it has a (low-power, daylight-readable, always-on) transflective screen.
According to their docs it increases the battery life by up to 3 days when you're not using the GPS.
Do you think they would work better if they were built into the laptop screen? Because that's what OP was raving about.
You can get batteries and external solar chargers (they are cheap and come in any size you want) if that's what you want to do. Building them into the device is STUPID. For one thing, you would need to put your device in direct sunlight to charge them ... and heat is not good for electronics. Also, the solar array size will be limited to whatever size your device is, that is: "too small".