As i keep thinking often
@Joe Shearer , Sir you have to teach us kids a lot of things.
Coming to the question of
@Nilgiri , it is rightly pointed out by Joe Shearer that locomotive is cost and here in context of repayment, we are actually comparing it with a locomotive that doesn't put back energy in system where it is being fed in first place. I'll try to put in terms of figures:
Let us assume it is 2015-16 (last year of WAP 4 and WAG 7 production). That time WAP 4 costed Rs. 7.9 Crores and WAP 7 Rs. 12.40 crores.
So let us take case of a WAP 4 and a WAP 7 produced that year. Assuming same running utilization (that is service hours) a WAP 4 would not have returned any power but a WAP 7 would @ Rs. 50 lakhs per year (this is an assumed value based on following formula
Rated power of WAP 7 4,500 kW
Number of days of operation in a year 300 days
Amount of time in day when using power 33%
Power Tariff (Averaged) Rs. 5.50 per Kw-H
Amount of power returned back 15%
Total amount of savings by power regenerated = 4,500*24*300*0.33*0.15*5.5 = Rs. 88, 20, 900.00 say Rs. 90 lakhs.
So in about 5 years loco would have earned around 4.5 crores that a WAP 4 would not have, had it been in operation in place of WAP 7.
These figures vary (especially that availability time on track) but can give you a rough idea of how economics of operation is like.