My citrus spent the winter in a dark basement. To give them some light, I installed a simple led flashlight and connected it with solar panel. Every day they received 1-2 hours of white light. Nearly no watering. No heating. The lowest temperature in the basement was about 0 °C, possibly -1 °C. The highest measured was about 6 °C. They spent three months underground.
Most of them did fine, except for the most frost sensitive like citron and lime. Satsumas, kumquats and ichangensis didn't lose a single leaf.