(a) In each municipality that has adopted the taxes authorized by this chapter, the taxes imposed by Section 321.104(a) and the tax imposed by Subchapter D, Chapter 151, are added together to form a single combined tax rate, except:
(1) in a municipality that imposes the tax on gas and electricity for residential use only the rate of the municipal tax is used to determine the amount of tax on the use, storage, or other consumption of gas and electricity for residential use; and
(2) only the rate of the municipal tax is used in a situation described by Section 321.205(b).
(b) The formula prescribed by Section 321.201(b) applies to the computation of the amount of use taxes under this chapter.