Measuring solar yield approaching winter solstice

Lol. I have unobstructed sky from dawn to dusk in storage. My expectation is that yield is about as good as it gets for untilted PVs. But I would like actuals rather than estimated numbers, if the cloud cover cooperates. I want to take enough amp hours out of the battery to get a pretty good actual charge reading. I probably will not be able to check up on the amp hours mid day.

Jim