And he uses massive amounts of insulation. He creates a large (big enough to walk in) wall cavity, and fills that with blown cellulose. I think it's on the order of twenty tons of cellulose. Not only does it insulate, but it forms a thermal mass.
Similarly for his foundation: a large sand base as I recall, then a slab, which create yet more thermal mass. The masonry stove at the building core is another, and finally a 5,000 gallon vertically-arranged water tank. This has a perforated distributor, and the idea is that he circulates water through his masonry stove and solar thermal panels whenever the output of those is warmer than the water at some point in the tank -- it tends to be ~120F near the surface, and ~40F near the bottom. The water stratifies according to temperature, and he banks his BTUs.