I know you don't want to use heaters for energy but just my bit- I only turn a mini oil one on in DD's room at about 3am when she wakes for her feed then turn it off when she wakes for a dummy at about 6am and the room stays a nice warmth.
Other than that, I put her in a singlet, long sleeve suitie, Snugtime bag, socks, cotton sheet and a wool blanket. This is usually enough and there haven't been many days where I have had to use the heater. Other times I will also put a cotton blanket on top of the wool one. I make sure I tuck the wool one in around her sides too.