The problem with solar panels is (1) they have to be fairly large for anything over 20W and (2) you need fairly ideal conditions to get to full power out of the solar panel, if it’s cloudy, raining, or heavy trees you may not get full power (or any power). Also you have to spend time getting the position just right so you get the maximum power and then the sun moves around on you. Strapping them to the bike and hoping for the best won’t produce good results.
I have a small 7w solar panel that is the size of a small tablet, but don’t use it that much. I use a SON dynamo with Edulux 2 dynamo lighting and a USB charger. It’s true that 3W only gives you 5V/600mA but this is enough to charge an in-line battery and keep the Garmin, iPhone and some other small devices such as backup battery lights charged for rides 300km or longer. Loss of 3W comes out to increasing the riding time about 10minutes per 100km for me at my long distance pace.