I agree, I would bring extra batteries.
However, lets consider what appears to be the single most popular brand of electronic devices here on BPL; the iPhone. I understand the allure of the iPhone. I have one. You can utilize an excellent GPS/Map program called Topo (very impressive). You can use it as an iPod or eReader and it has a Kindle application. It has a camera of so-so resolution. Also has a pretty good Web browser. You can text with it. You can even call someone with it :)
The battery is not replaceable, so you cannot bring a spare. You can purchase a battery pack with 4 AA cells to recharge the iPhone. But you still have the same problem, what do you do when you use up your battery pack.
There a 3 strategies with solar.
#1 - extend the battery life. That is, if it is going to go dead in lets say 4 days, you can use solar to extend it for 7 days.
#2 - replenish the amps you use each day to get it to a full charge by the end of the day.
#3 - reserve battery bank. That is charging an external battery pack so that if you run into a week of solid rain, you can use your reserve battery pack.
Under optimum conditions a Goal Zero Nomad 7 will recharge an iPhone in about 2 hours. It will also recharge the battery pack in about two hours. The charge controller has a USB port for your iPhone (5 volts) and a different port for the battery pack (6+ volts). Charging while hiking presents other problems such as orientation and shaded areas. But lets say under these conditions you can charge either in 4 times as long, which would be 8 hours.
So lets consider a thru hiker who may carry a camera, GPS, and a cell phone. Average weight of each is usually around 154 grams or 5.4 ounces. Add spare batteries for each; lets just say another 151 grams total (about what my Garmin and Canon camera weight - actually they are a little more). Now we have a total of 613 grams or 21.6 ounces. If you are thru hiking you may need a charger for your camera batteries and cell phone, lets say 68 grams each for the plug in the wall chargers X 2 = 136 grams or 4.8 ounces. Total weight of this thru hike kit is 751 grams or 26.5 ounces. Plus you now need to go into towns and find access to the grid to charge your stuff up. Okay there might be more efficient ways, but maybe this is what a lot of thru hikers use.
Now lets consider a Goal Zero set-up with an iPhone.
Solar Panel (quality noncrystalline) = 379 g
Battery Pack with cable = 181 g
iPhone with cable (remember enhanced capabilities) = 181 g
Total = 741 g or 26.1 oz
Solar net savings = 10 g or .4 ounce.
Also if you head light uses AA batteries, then you have spares from the battery pack. The battery back also has a pretty bright LED flashlight.
Will I ever do this? Probably not. But for some people it might be a viable option. If I were to do the PCT (which I may when I retire), I would take a serious look at this set-up.
Plus we are not buying throwing away AA batteries for some of our devices, which is an environmental issue, as is using the grid to recharge our batteries on zero days -- if that is a consideration.
Food for thought.