irrigations per year to obtain the annual quantity of electricity used
for irrigation. The annual number of irrigations per year is a user
specified input variable to the program.
 Net generation of the photovoltaic system is calculated by sub-
tracting the annual electrical demand for irrigation from the annual
total electrical output of the system. It should be noted that measuring
net generation in this manner differs somewhat from the theoretically
"correct" measurement as implied by equation (5). The error resulting
from this approximation in the calculation of net generation can, however,
be shown to be very small.

 Economic and Institutional Component

 The function of this component is to estimate the change over time
in the economic variables used in calculating and comparing the discounted
energy cost of photovoltaic powered and conventional electric powered
irrigation systems. The basic input variables to this component are the
escalation rate of electricity KWH charges, the discount rate, the price
utilities must pay for electricity purchased from dispersed systems, and
the rate of decline in the cost ($/KWH) of photovoltaic arrays. All
prices and costs are expressed in terms of 1980 dollars and all rates of
change are in real terms. Furthermore, it is assumed that time-of-use
(TOU) electricity rate structures are in effect.
 Electricity rates are assumed to escalate in an exponential fashion
with the real annual escalation rate specified by the user. The 1980
base price of electricity is set at $0.03604 per KWH. This price repre-
sents the off-peak price of purchased electricity. Under the assumption
that all irrigation occurs off-peak, the annual cost of electricity for
a conventional system is obtained by multiplying the annual quantity of
electricity used for irrigation by the corresponding "forecast price".
The rates utilities must pay for electricity purchased from dispersed
systems is assumed to be proportional (via the buy-back ratio) to the
price at which utilities sell electricity. The buy-back ratio is a user
specified variable which reflects institutional decisions regarding the
precise definition of avoided cost. Thus a buy-back ratio greater than
one implies that avoided costs are realized during peak periods.