(a) Unless it is shown to the satisfaction of the commission that it was reasonable to choose not to consolidate returns, a public utility's income taxes shall be computed as though a consolidated return had been filed and the utility had realized its fair share of the savings resulting from that return, if:
(1) the utility is a member of an affiliated group eligible to file a consolidated income tax return; and
(2) it is advantageous to the utility to do so.
(b) The amount of income tax that a consolidated group of which a public utility is a member saves, because the consolidated return eliminates the intercompany profit on purchases by the utility from an affiliate, shall be applied to reduce the cost of the property or service purchased from the affiliate.
(c) The investment tax credit allowed against federal income taxes, to the extent retained by the utility, shall be applied as a reduction in the rate-based contribution of the assets to which the credit applies, to the extent and at the rate allowed by the Internal Revenue Code.