The thermal properties and the price of the collector can be included in the model
by de®ning them as life-cycle savings (i.e. net present worth). The life-cycle-savings
are the dierence between the life-cycle costs of a conventional fuel-only system and
the solar system. The life-cycle cost is the sum of all the costs associated with an energy delivery system over its life or over a selected period of analysis, in today's
money