The commission may require a public utility to defer inclusion of all or any portion of the reasonable value of property determined not currently used and required to be used and may require the phase-in of such value over any period of time and in such increments as it determines to be appropriate. If the commission requires a public utility to defer the inclusion of any portion of such reasonable value and orders a phase-in of such value, it may exclude any or all of the carrying or finance costs incurred after the date of its determination and throughout the period of any deferral or phase-in as so ordered.