Are you wanting to know how many watts your peltier will be able to pump when running at 13.8V instead of 15.8?
If so, my calculations sounds about right. At 13.8V, you are running at about .8 of the rated voltage, which would give you somewhere around 103 watts. To be more specific, my TI spurt out:
103.063291139240506329113924050612 watts.
Does this sound about right to everyone else?