k k k
 VCECt = P E (2)
 t t t
where
 k
 VCECt = variable energy cost of a conventionally powered system in
 year t
 k
 P = unit price of fuel type k in year t
 t
 k
 E = amount of energy type k used for irrigation in year t
 t

The discounted cost of an investment in an irrigation system powered by
fuel k can be written as
 k T+j
 DCC. = SC + PPU. + E D (VCEC ) j = 1,...,J (3)
 S3 J t t
 t=j

where
 DCC = discounted cost of a conventional irrigation system pruchased
 in year j
 D = discounting factor, D = (l+r)t and r denotes the real
 t t
 discount rate
 Equation (3) provides a means of computing the discounted total cost
of irrigation over a period of T years assuming the initial investment
is made in year j, j + 1, and so on. While the theoretical construction
of equation (3) is relatively simple, obtaining values for the included
variables is a more difficult matter. Consider for example an invest-
ment life of 20 years (T = 20) and the investment can be undertaken in
any of 20 succeeding years (J = 20) beginning in 1980. Fixed investment
costs must be estimated for the next 20 years and energy prices and the
annual amount of irrigation must be estimated for the next 40 years.
Thus, the variables in equation (3) could have many different values
depending on one's forecasts about future prices.

 Photovoltaic Powered Irrigation System Costs


 Although the fixed cost structure of a photovoltaic powered irri-
gation system is similar to that of a conventionally powered system, the
variable energy cost structure is considerably different. The difference
is that a photovoltaic system can produce electricity. During periods
of time when irrigation is not needed, the electricity produced by the