It’s free, and the most environmentally-friendly home heating option available.
You’ll need large northfacing windows to allow the sun to shine in during the day, a large thermal mass such as a concrete floor to store the heat, and insulation so the heat isn’t lost too quickly at night.
And you’ll probably still need other heat sources for the coldest days.
If you’re designing a new home, incorporating some passive solar heating into the design may not cost very much at all. It may also be possible to include some solar features when you’re renovating, but it’s harder.