system can be sold to the utility.5
 The fixed cost investment for a photovoltaic powered irrigation
system purchased in year j can be written as

 ICS = SC. + PPU3 + PC. j = 1,...,J (4)

where
 ICS. = fixed investment cost of a photovoltaic irrigation system
 purchased in year j
 SC. = cost of water dispersal system structures purchased in year j
 3
 PPU. = cost of electric pump-power unit purchased in year j
 3
 PC. = cost of a photovoltaic system purchased in year j

 The similarity of fixed investment costs between conventional and
photovoltaic powered systems can be seen by comparing equations (1) and
(4). If the conventional system is electrically powered, the fixed cost
structures are identical with respect to water dispersal system structures
cost and pump-power unit cost, differing only in that equation (4) con-
tains the additional fixed cost of the photovoltaic array. Even if the
systems are powered by different types of energy, the water dispersal
system structures cost will be identical. Therefore, the economic
feasibility of photovoltaic systems depends on whether the decrease in
variable energy costs and surplus electricity sold to the utility can
offset the additional fixed cost of the photovoltaic system.
 The variable energy costs of a photovoltaic irrigation system can
be expressed by

 VSECt Pe Ee RP SAG (5)
 t t t t t
where
 VSECt = variable photovoltaic system energy costs in year t


 Electricity produced by the array can also be used for other on-
farm activities. In this report it is assumed that all surplus electri-
city produced by the photovoltaic system is sold to a utility grid.

 This assumes that comparisons are being made for similar types
of irrigation systems (e.g., permanent overhead, etc.). No attempt is
made in this analysis to compare different types of irrigation systems.