Up here in Fairbanks, AK, we don’t get much daylight this time of the year (sunrise ~11am, sunset ~2:40pm), and the sun is so low on the horizon that at solar noon, on a perfectly clear day, we may get 17 W/m2. So, I am quite impressed that my Tempest is still going strong with a battery voltage still at 2.36 V. On a clear day, it might not be charging the battery, but it looks like it’s keeping even. In a month, we’ll already have quite a bit of more daylight, and I’m sure that the battery will be charging then.
My only issue so far, is that when it gets cold (-30F), the windspeed sensor seems to be out of wack, showing wind speeds peaking at 45 mph, but the trees branches are not even moving.
Other than that, quite happy so far.
Happy New Year y’all!
Interesting confirmation that the solar reading might read low while the solar panels may be charging better than the solar radiation suggests. The 17 w/m2 is measured on the horizontal sensor. But the solar panels are vertical to catch a better angle including for many people the sun rise and set period. With the sun so low the panels are at their perfect angle to face the sun. 
Update:
Today (2 February), the battery became fully charged (2.83V). The lowest the battery went was 2.35V at the end of December. It struggled for a while with very low sun angle, but never quit. After 3 very sunny days (the last 3 days), the battery fully recovered. So, I was skeptical at first it was going to make it through the winter, but it proved me wrong. Success!