[justme1969]

We can estimate payback in 2 years from your figures and less if you also take in account the life cycle expense on alternator and loss of effective power from alternator draw especially during accelleration activations.
looks a little better this way.

Then for my application Ill take into account key off coasting in dark that saps my system.

So long stories short if you plan to keep your car more than a year it may just pay.