A solar panel the size of a typical rear rack would probably be about 1/3 square foot. Solar cells put out about 10 watts per square foot, so you're looking at 3.5 watts at best, in full sunlight with the sun hitting the panel at a right angle. This will never occur. Also, the panel will be in the shadow of the seat/rider about half the time, even if you ride in an area with no shade all the time. I'd guess at best that you're going to see a net average output during the daytime of perhaps 1 watt.
Now, with a dynamo you're using the energy as it is produced, so you don't pay the penalty of storage. With a solar panel the energy has to be stored. Charging systems and battery together will be perhaps 50% efficient if you buy really good one. So now you're down to 1/2 watt. If you have a modest 3 watt LED system (front and back, not super bright), you will have to be in the sunlight 6 hours for every hour you want to run your lights.
And it's not bright and sunny every day either. You'll still need the ability to charge your system from mains power.
It's a laudable idea but I think the idea of portable solar charging is not a great one. I think you'd be better buying a modest 50 watt panel for your home, charging a 12v battery there all day, and using it to charge your bike batteries at home every day. It would still be pretty cheap and you wouldn't have to carry that stuff with you.
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Work: the 8 hours that separates bike rides.