Do you have stats on how many watt hours you collect from the 100w panel in the Dec/Jan (assuming you’re in the northern hemisphere),

You can model average insolation (and use that to extrapolate average harvest) by month using tools like PVwatts. Here's a walkthrough.

Using Bowling Green, KY as an example since it's on the 37th. 100w of flatmounted panel on an MPPT solar charge controller would average:

Solar wattage	100
Month	Daily Wh Avg
Jan	168
Feb	249
Mar	331
Apr	426
May	513
Jun	598
Jul	555
Aug	506
Sep	396
Oct	305
Nov	201
Dec	156
Average	367

Derate those yields by ~18% if using PWM. <-- rule of thumb, not gospel

if we are on the west coast instead, here's Santa Cruz, California:

Solar wattage	100
Month	Daily Wh Avg
Jan	206
Feb	286
Mar	386
Apr	519
May	582
Jun	642
Jul	605
Aug	542
Sep	447
Oct	349
Nov	245
Dec	183
Average	416

and how those stats vary on overcast days vs sunny?

The figures above are daily averages, including normal weather patterns and are how we size our systems. But for the sake of curiosity/understanding, my observations have been that if my clear-day harvest is X then overcast is 0.6X, bright overcast is 0.7X and dark/rainy is 0.05-0.10X. Cloud-edge effect and other reflective phenomena can result in harvest >1X.

While it does freeze here (occasionally down to 0° F) the battery will be inside the chicken coop where the temp has always remained above freezing.

A battery warming solution could be implemented for $20 (warming pads, 12v temp controller).