November and December tend to be overcast here, too. Somewhere in a passive solar heating book in my personal library, I found a table listing hrs of insolation for each month by location. They only listed Burlington, which is 2 hrs. away from me, but it was in the neighborhood of 20 hrs for each (Nov. & Dec.)
The big hurdle I’m grappling with as a new high tunnel owner wishing to grow figs and other stuff that would theoretically benefit, is what happens later in winter when the sun DOES come out in force. A sunny average 20 degree day easily raises our 32x48 solawrap encoded tunnel to 85 degrees. Then at night, the temp in the tunnel tracks outside temps closely, lending only a modest advantage. Our low this past year occurred mid-Feb. It was -18 F, and our high low thermometer in the high tunnel logged -4. It’s not clear to me that ANY fig is going to be able to endure these kind of swings, so I may be locked into growing in containers that I can move to be wintered somewhere dark and cool.
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what I’ve done is, I have several solar panels and a jackery -type battery. I use heat mats under the pots, on a metal tray, and during the day, hot lights. that keeps it warm all winter and there’s just enough light to charge the battery through the day and keep the mats going at night. I will occasionally run a rated extension from the house and bring a small space heater out with me if I’m working in there and that also helps. I have so much insulation on the thing that it really stays warm enough through cold nights
once it starts to get sunny I will only run the mats at night. if it gets over 65F on a sunny day I’ll run the fan on the heater with no heat and crack the door. it’s enough to keep it between 40/ish and 65F all winter.
this time of year the mats turn on at dusk.
I also use a shade cloth to keep the figs covered. if they stay in the dark they’re less likely to wake because it’s a bit warm. I took those off in mid March as it stopped getting to our freezing icy time of year.
it uses only the power from the battery and panels, except for one night that was forecast heavy wind and I think in the teens, I set up the heat out there and left it on blast with a temp regulator to turn it on if it got below 40 degrees. it ran a few hours that night.
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took down all the insulation and will be taking all of it down next week- putting up a shade cloth on one side, and done for the season. is already up to the 80sF outside and up to 100F inside it in the day with door open and a fan running. so it’s got to come down right after last frost.
still overnight with anything tender, my starts on a wagon in and out of it every night and morning. worst time of year really because of it.