I don't have one, but I think (may be I'm incorrect), you have to recover the cost in 8 years.
Usually
Imagine you have solar
if you pump fuel for 10,000/- then 10,000 x 365x 8 = 29 M or 5000 fuel per day = 14.5 M
having solar, this is pure profit
But if you pump fuel 5000 per week; 5000 x...