Making the most of a site’s solar potential is step one in the design process, coming even before any aesthetic sketches are developed. Orienting a building toward the sun, specifying the size and type of windows, and balancing light and thermal mass are all elements of passive solar design. Unlike solar hot-water systems or photovoltaic panels, which are active or mechanically controlled solar components, passive solar design relies on nothing more than good planning and design to help make the building more energy self-sufficient.
From David Johnston’s Green Diary
One of David’s first projects for a production builder was to take a model the company had built hundreds of times and "green it." I was stuck with the floor plan — that couldn’t be changed — but not the way the house was oriented. One wall of the house was mostly glass, and this typically faced west to make the most of mountain views.
The first thing I did was to rotate my project house 90 degrees so the glass wall could do a better job of capturing sunlight. Along with modifying the standard type of window glass the builder was using and extending roof overhangs, this simple change reduced the heating and cooling load by 40%. As a result, we started with a building that was 25% more energy efficient than government Energy Star standards.
Even in winter, rooms with south-facing windows can be warm and comfortable. The key is balancing window area to relative mass of floors and walls illuminated by sunlight.