
86 Principles of Sustainable Energy Systems
Solution
Step 1
Compute the PWF using Equation 2.5 or Table 2.1 for the discount rate of i = 10%
PWF( ,) †
.10 1
1
0 909
1
=
+
=
PWF( ,) †
.10 2
1
0 826
2
=
+
=
PWF( ,) †
.10 3
0 751
3
=
+
=
Step 2
Compute the present worth of each opportunity benet ow by multiplying the PWF
by the annual benet amount. Note that we use the same signicant gures to report
results as was given in the cost and benet estimates.
Year PWF
Annual
Benet ($)
Present
Worth ($)
A B A B
1 0.909 100 0 91 0
2 0.826 150 225 124 186
3 0.751 200 225 150 169
Total 450 450 365 355
Note that although the total benets are the same, the prese