P = unit price of electricity in year t
 t
 Ee = electricity purchased to supplement array production in
 t
 year t
 RPt = resale unit price of electricity paid to dispersed producers
 for electricity in year t
 SAGt = quantity of surplus electricity produced by the photovoltaic
 array and sold to the utility in year t
The variable energy cost structure of a photovoltaic system differs con-
siderably from that of a conventional system. The first term on the
righthand side of equation (5) reflects the fact that a photovoltaic array
cannot supply all of the electricity necessary to power the irrigation
system at all times. This reflects the dependence of the output of a
photovoltaic system on incoming solar insolation levels (see Appendix A)
and the fact that irrigation may also occur during the night. In the
absence of battery storage of electricity, power must be purchased from
the utility to supplement the array power.
 The second term on the righthand side of equation (5) represents
the revenue from sale of surplus electricity to the utility. This sur-
plus electricity is generated during time periods when no irrigation
occurs and, during irrigation, whenever the photovoltaic array produces
an amount of electricity greater than that required to power the pump
motor. If the value of this resale of surplus electricity is greater
than the value of purchased supplemental electricity, the variable energy
costs of a photovoltaic system will be negative.
 The discounted cost of a photovoltaic powered irrigation system
purchased in year j and having an investment life of T years can be
expressed as the sum of equations (4) and (5).
 T+j
 DCS. = SC. + PPU + PC. + D (VSECt) J = 1,...,J (6)
 3 3 3 3 t=jt

where



 7The use of battery storage of electricity is not included in
the simulation model.