The estimation of discounted costs for conventionally powered and
solar powered pump-power units over some period of time requires a number
of assumptions regarding the behavior of energy prices and other economic
variables over time. Give that photovoltaic arrays are also dispersed
generators of electricity, issues involving the resale of electricity and
utility pricing policies must also be considered. The following discussion
outlines the basic manner in which the discounted pump-power unit and
energy costs are utilized to evaluate the economic feasibility of photo-
 4
voltaic powered irrigation systems.

 Conventional System Costs


 The calculation of the discounted cost of a conventionally powered
irrigation system is straightforward. This discounted cost of an irri-
gation system with an investment life of T years is composed of a fixed
investment cost and a stream of variable energy costs.
 The fixed cost investment of a irrigation system purchased in year
j can be represented by

 ICC = SC + PPU j 1,...,J (1)

where

 ICC = Fixed cost investment in year j

 SC = cost of water dispersal system structures purchased in year j

 PPUk cost of pump-power unit using fuel type K,

 k = g (gasoline), e (electricity), d (diesel), 1 (1-p gas)
 purchased in year j
 Annual variable energy costs for a system using energy type k, k = g, e,
 d, 1 are given by

 and the fact that the use of photovoltaic arrays involves no change in the
 way irrigation is practiced, the economic feasibility of these systems can
 be assessed by only examining costs relating to the pump-power units and
 commensurate energy costs.

 A detailed discussion of the measurement and calculation of these
 costs are contained in Appendix B.