For example, today the sun is out, but we had a high of 46 degrees outside. Inside we are toasty warm in the low 70s with no forms of heat needed.
You must also start out with the right kind of insulated slab to hold the heat in the floor, instead of it leaking out into the ground. This is essential!
The walls must be well insulated, and then we chose dark gray tile throughout the house to absorb the heat as it enters the house from our south-facing doors and windows. Right now the sun is shining about 10 feet into our home!
Fortunately we were able to find the perfect passive solar perch for our new home, one that faces south and also offers us a 180 degree view of the Spanish Peaks and the Sangre de Cristo mountain range.