How many watts are required for a self-sustaining house that relies on sunlight for heating?
We like your approach of creating a building which does not require any conventional heating and cooling methods. It sounds like you are planning to create a “zero-energy house.” Zero-energy buildings have been created in the past in Europe, but only in a small quantity since they have been very expensive.
In Germany, various strategies and technologies have been tested in the past 10 years, and the most successful is the so-called "Passive Haus." The Passive House Technology has been used in hundreds of residential and commercial projects throughout Europe. This established building technology enables residents to reduce their home's energy consumption by up to 90% while at the same time creating a comfortable building.
Passive buildings are using the following improved basic construction materials/methods:
• More insulation (super-insulation without thermal bridges)
• Better windows
• High air-tightness
• Heat/cool recovery from exhaust air
These small measures result in a drastic reduced demand for heating and cooling of the building. Passive buildings have a high thermal comfort and a very good indoor air quality.
You were asking how many watts are required, and the magic number for this building type for the calculation is 10W/m2. Another good resource for you might be the U.S. Passive House Institute.