of using photovoltaic arrays to power irrigation systems became less
than the discounted cost of purchasing electricity for powering irriga-
tion systems before the year 2000 (Table 3).


Table 3.--Estimated first year in which photovoltaic
 irrigation systems are economically feasible


 Buy-back ratio
Scenario 0.25 0.50 0.75 1.0 1.25 1.50

Optimistic 1990 1987 1986 1985 1985 1984
Base 1997 1985 1993 1992 1991 1990
Pessimistic 2000+ 2000+ 1998 1997 1995 1994



 Simulation results for the optimistic scenario, which assumed a
rapid decline in photovoltaic system costs and a significant increase
in electricity rates, indicated the first year of economic feasibility
could occur as early as 1984 if the buy-back ratio was 1.50. Generally,
each increase of 0.25 in the buy-back ratio moved the initial year of
economic feasibility toward the present by 1 to 2 years. Increasing the
buy-back ratio from 0.25 to 0.50 changed the initial year of feasibility
from 1990 to 1987 whereas increasing the buy-back ratio from 1.25 to 1.50
changed the initial year of economic feasibility from 1985 to 1984. The
estimated differences in the discounted costs of photovoltaic powered
and conventional electric powered systems assuming the investment is
made in succeeding years from 1980 to 2000 are plotted in Figure 2 for
several buy-back ratios.
 The base model scenario was characterized by a moderate decline in
the cost of photovoltaic systems over time. Similarly, the annual
increase in utility electric rates was also moderate, increasing at an
annual real rate of 2 percent. As with the optimistic scenario, as the
buy-back ratio increased, the first year of economic feasibility moved
closer to the present. With the buy-back ratio at 0.25, the initial
year of feasibility was 1997, whereas a buy-back ratio of 1.50 resulted
in photovoltaic systems being economically feasible in 1990. In general,
each 0.25 increase in the buy-back ratio moved the initial year of