exact rates utilities will pay for electricity from dispersed systems
unfortunately is still unclear [Norman, 1981].

 Economic Feasibility Criterion

 Equations (3) and (6) give expressions for obtaining the discounted
cost of purchasing and operating a conventionally powered and a photo-
voltaic powered irrigation system for T years assuming the initial
investment occurs in year j, j=l,...,J. Photovoltaic systems would be
an economically feasible investment in year j, if

 DCS. < DCC j = 1,...,J (7)

The first year of economic feasibility will be that year in which the
discounted cost of a photovoltaic system is less than or equal to that
of a conventionally powered system. Substituting for DCS. and DCC
in equation (7) yields
 T+j
 SC + PPU + PC. + E Dt(VSECt) < SC. + PPU. +
 t J J t t 3 3
 t=j

 T+j
 E Dt(VCECt) (8)
 t=j
where all terms retain their original definitions. Upon simplification
and rearranging terms, equation (8) can be rewritten as


 k e T+j
 -PC. + (PPU PPU.) + E D (VCEC VSEC ) > 0 (9)
 3 t=j t t t -
 t=j
 j = 1,...,J
In this form, the importance of the revenue earned from the sale of
surplus electricity can be seen. Given the intermittent nature of
irrigation, the sign of VSECt (equation (5)) is almost surely negative.
Thus, the effect of this term in equation (9) is to offset the additional
fixed cost (-PC ) attributable to the photovoltaic system.
 Equation (8) also facilitates the discussion of several additional
aspects involving photovoltaic system feasibility. First, the effects
of declines in the cost of photovoltaic arrays can be seen. The only