Successful solar charging experiment today! From my studies of renewable energy, I have long known that solar panels (also known as solar photovoltaic, solar PV for short) will charge even on an overcast day,* at least on a somewhat bright overcast day.
Here we have a demo of that distinction in action. A small red light on the back of the panel shows that enough solar energy is hitting the panel to charge.
Since the clouds are actually pretty thick, I wasn’t expecting the charge to work at all. It was kind of cutting in and out, but some charging is happening for sure. As indicated by the battery indicator on my handy little generator.
The number on the indicator light shows the wattage that the solar panels are taking in. As you can see from the photos, the wattage can fluctuate from one second to another as the light conditions vary.
Of course, the panels work better the more direct sun there is. It would take a very long time to charge up the generator battery all the way from 0% to 100% in the type of cloudy conditions we have here right now.
The generator is my power source for my mini off grid studio/she-shed/glamp-partment (which would be the garage, taken up by a car, except that I have the good fortune to be able to live without having to own a car). 
*(Unlike the solar oven, which requires direct sun to do its work of cooking. The solar oven, also pictured here, is closed until the sun comes back out and until I want to cook/heat something.)
Update: The clouds got cloudier, so I moved the unit indoors to charge from the wall. The last two photos show that wattage coming in from 120-volt AC plug.