(b) The maximum total credit allowed for an alternative-fuel motor vehicle is:
(1) For a vehicle with a gross vehicle weight of not more than ten thousand pounds, three thousand seven hundred fifty dollars;
(2) For a vehicle with a gross vehicle weight of more than ten thousand pounds up to twenty-six thousand pounds, nine thousand two hundred fifty dollars;
(3) For a truck or van with a gross vehicle weight of more than twenty-six thousand pounds, fifty thousand dollars; and
(4) For a bus capable of seating at least twenty adults, fifty thousand dollars.
(c) Subject to the limitations set forth in subsection (a) of this section, a taxpayer who is otherwise entitled to a credit against tax who claims the credit provided for in this article on the basis of any alternative-fuel motor vehicle that operates exclusively on electricity is entitled to an additional credit of ten percent of the credit which is otherwise allowed under subsection (b) of this section.
(d) The maximum incremental credit allowed per year is one third of the credit attributable to five vehicles with the cumulative credit over a three-year period not to exceed one third of the credit attributable to fifteen vehicles.