I agree with you, sunny day, cloudy day, percents, etc.. mean nothing here for any type usable basis.
I have one pannel. On a bright clear day it made 100% of the power it would make on a bright clear sunny day. High volts, good amps.
But 2 days later we had a storm nearby and it was DARK because of thunderstorm cloud cover all day. Those really nasty thick purple black clouds, about 2pm (afternoon/daytime) it looked like 10pm (late night/dark time), and all day was that way though sometimes it was not raining.
My pannel made 0% power that day. I know this because about 11AM when I should have been making the most power I could make that day I disconected the pannel and connected a test wind genny becuase I was gettting winds and it was turning well. At that best time of day my pannel read less than 12V open, It was darkout so I expectd nothing and that's what I was getting. I was below battery charging volts and that was about the best sun light of the day. This was about a 35-50watt pannel, unsure the brand.
So I went from 100% on a good day to 0% on a real cloudy day within about 2 days.
I think this would be a good average for about any brand of pannel and pretty acurate testing of highs and lows I can expect based on cloud cover in my area now and then
Percentage of cloud cover will probably effect percentage of power, and I have come to the conclusion that the percentages pertain in reverse order to some very rough degree of which I am not yet certain.
It would seem at this time that 0% clouds equals 100% power,
But 100% clouds equals 0% power. I have not test the power cruve ratio like 25/75, 50/50, or 75/25 to see if the ratio holds steady, pretty shure it would not. But 0/100 and 100/0 is correct ratio for highs and lows.
However this is not totally acurate and could vary with voltage of battery bank being charged also. I was getting 0% into a 12V battery, however I did have some lower volts, perhaps I could have been charging a 6V battery at .5 or .25 amps??
Certainly at 5-7V pannel output I could have been charging a bit into my sub-C 1.3V cells.
In such case of lower volt battery bank, I may have been getting 0.2% or 0.05% power?
I did have a tiny 12V fan that starts up around 2-3V, just a few watts, and it would run very slowly at that time of 0% so there was some amount of power, though not usable to me right then other than a tiny fan or some leds maybe.