[Minn]

Even on cloudy days solar heat is available and a well insulated building could probably use it as a sole means of heating most of the time.

You must live in the tropics.

[count-your-change]

No at all. Space heating is the easiest use of solar energy even in below zero weather.
There are lots of designs available on the inter-net but one of the less complicated involves using solar heated water and using the stored hot water in a radiant heat floor system.