An off grid way to judge how cold it got the night before

Living off grid we rely on 12v battery power and right now we are only using 2 deep cell marine batteries. While they work great, we are really limited on power usage. (only around 300 watts available) We do have a small solar panel to handle battery maintance, any type of a draw will bring those batteries down quickly. To recharge them we rely on generators which need to be manually started. All that to say, our heater is forced air running on 12v and at night it has been running on average two to three times an hr. Last night it must have been on every five min! I checked our batteries this morning and they are almost 40 % discharged! (in 7 hrs!) Yep it was a cold one last night.
On the good side, clear skies and the solar panels are making quick work of replinishing the power used last night.
Just another reminder that life, off grid, is very different than life on!