Why is it called "passive" solar?
A "passive" house achieves overall energy savings of 60-70% and 90% of space heating without applying expensive "active" technologies like photovoltaics or solar thermal hot water systems. Energy losses are minimized, and gains are maximized. Superinsulation and air-tight construction minimize losses. The heat/energy recovery ventilator helps keep energy that has already been generated in the house instead of venting it out. Knowing about thermal storage capacity of certain materials and their "passive" effects on the indoor temperature of a home, the architect/designer can plan for enough thermal storage mass in a house by specifying tile floors, finished concrete slabs, concrete or granite countertops, stone fireplace surrounds, adobe walls or earthen plaster (in a passive house thermal storage mass no longer has to be painted black or directly exposed to the sun! The PHI recommends 5-6 thermal storage surfaces per room for optimal effect). In climates with low humidity and high day/night temperature differentials, it can be very effective to bring in outside air through a pre-warming/cooling "Earth Tube" to passively harvest the energy of the earth (without use of active ground source heat pumps).
It is beneficial to install solar hot water systems in addition to the passive house construction techniques. Next to space heating/cooling, domestic hot water is the biggest energy requirement in a home. Solar hot water can also effectively be used to provide the remaining space heat wherever heating is necessary (hydronic heat coil integrated in supply air of the ventilation system).