Depending on your needs, I think you will be hard pressed to heat only with solar or steaming compost.
Solar requires thermal mass and added insulation.
I've got a string of nights here in the 20's, so I'll see. I've been adding both thermal mass and insulation. I do have a heat lamp on a timer... It's my first winter with the "solar cabana", so much remains to be done.
With that said day time temps have been a delight inside with outside temps in the the 30's.
No objection to any form of heating, but insulation is often overlooked.
Jeff
Have you