(1) At the discretion and option of the local tax authority, the tax authorized under s. 166.231 may be levied on a physical unit basis. The tax on the purchase of electricity may be based upon the number of kilowatt hours purchased; the tax on the purchase of metered or bottled gas (natural liquefied petroleum gas or manufactured) may be based on the number of cubic feet purchased; the tax on the purchase of fuel oil and kerosene may be based on the number of gallons purchased; and the tax on the purchase of water service may be based on the number of gallons purchased. (2) In the event that a municipality chooses the option provided in this section to tax on a physical unit basis, the municipality may choose not to levy and collect the tax authorized under s. 166.231 on any amount up to, and including, the first 500 kilowatt hours of electricity per month purchased for residential use. Such exemption shall apply to each separate residential unit, regardless of whether such unit is on a separate or a central meter, and shall be passed on to each individual tenant. (3) In exercising its option pursuant to this section, each municipality levying a tax pursuant to this section shall implement a new tax rate structure and tax base in accordance with this act. The new tax rates shall apply to prior purchases of service if the purchases were billed during the month of implementation and thereafter. The shift in the tax rate and tax base for electricity, metered or bottled gas, fuel oil, kerosene, and water shall be accomplished in the following manner:
(a) Each municipality levying the tax shall, prior to converting to unit-based rates, compute the amount of tax it received from each source for the most recent 12 months for which such data is available.
(b) The amount determined under paragraph (a) shall be divided by the number of units purchased and taxed for the same period of time used in paragraph (a).
(c) One hundred five percent of the resulting figure rounded to no more than four decimal places shall be the maximum amount per unit which the municipality may levy upon converting to unit-based rates. However, during the year of conversion to a physical unit tax, the municipality may adjust its rates to ensure that revenues derived from the tax shall equal 105 percent of the revenues derived in the immediately preceding year. In those years subsequent to the year of conversion to a physical unit tax, the municipality may amend its tax rate by ordinance.