Buildings which employ passive solar design principles seem to fall into three general categories; pre-passive, passive and post-passive (refer to the table shown below). In pre-passive solar buildings the south facing window area is pretty typical of current construction practices. As a result, the cost of the building is also very typical of a conventionally constructed structure. There are however some design considerations that need to be followed to double the contribution of energy provided by the suns warming rays (a conventional building may only take in 25% of its heating load from solar radiation).

To move up to the next level one needs to increase the amount of south facing window area. One also needs to add thermal mass to the structure to prevent it from overheating. This raises the cost of the structure but it also increases the solar energy contribution. The heating load is thereby reduced.

In a post-passive solar building the amount of south facing window area is even greater and more thermal mass is required, but almost all of the heating load is provided by solar radiation. The building cost is somewhat higher.

A word of caution! The biggest mistake folks make, even some professional builders and architects, is that they put in too much window area for the climate. This may cause an uncomfortable living environment, even during the winter months, and certainly drive up the air conditioning costs during the summer months. Once the building is built its expensive to go back and change the design. The balance of all six of the solar design principles; total window area, south facing window area, the degree of building insulation, window shading, the amount of fresh air exchanged with the outside, and the thermal mass of the building are all imperative to "getting it right" to obtain the desired effect.

Level	South Facing Window Area (%)(1)	Solar Energy Contribution (%)(2)	Increased Building Cost (%)	Benefits	Constraints
Pre-passive	up to 7%	up to 55%	-	any architecture lowest cost	lowest solar energy contribution
Passive	up to 12%	up to 75%	up to 3%	higher solar energy contribution	more architectural constraints higher cost
Post-passive	up to 21%	up to 95%	up to 15%	highest solar energy contribution	most architectural constraints highest cost

(1) South Facing Window Area is defined as the square feet of window area per square feet of adjacent floor area expresses as a percentage.

(2) Solar Energy Contribution is defined as the amount of energy passing through the windows as compared to the required heating load of the structure expressed as a percentage.

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