Heating an enclosure to -20°C

[mauroh]

Keep in mind also the "cold start"...
We had some issues with products specifically designed for extreme temperatures that can "keep working" just heating themselves inside a badly insulated enclosure, but failed miserably if were powered on at low temp.
We solved it with a circuit totally independent (if the microcontroller is non starting also the heater will not be powered  ) that will rise the circuit temperature up to a threshold before releasing the reset.
Mauro

[Rigby]

I was thinking of putting an LM7805 right behind the spark and having it dissipate 12V down to 5V, or even 3.3, but the current consumption is so low, I'm not sure if it would help at all.

Timb: I know a few folks at spark as well. The more I learn about you, the more keen I am to befriend you.

[kaindub]

Solar powered
In winter

I think you have a challenge to even supply power to your project, let alone keep the electronics alive.

I'd do a power analysis first.
See what power you can harvest from the solar cells. Will they work at those low temperatures. Then the batteries. They lose capacity quickly at low temps. If it's a cell with a fluid composition then the fluid will freeze at those temps.

Robert