I have no idea the answer to your question. I suppose it would depend a lot on HOW cloudy and for how long. However, I have observed that those little solar lights you push into the ground to light pathways seem to charge up as well on cloudy days as sunny ones. We have some at our cabin which is in the woods, and I’ve been surprised how brightly those little lights shine all night long, which means that even in shaded areas they get sufficient light to charge them.
After I posted, I googled it to see what was there, and it pretty much said what I thought.
They will come in anywheres between 20-50% of normal efficiency.
Which is way better than nothing, I mean I live in Western Washington where the sun can be pretty shy for a long time!