Rates set in accordance with estimates of annual expenses

(1) The officer referred to in ORS 225.220 shall annually before January 1, make a written estimate of the probable expense of maintaining and conducting the electric light plant or distributing system during the next ensuing year, including the cost of any contemplated alterations, improvements, additions or extensions, together with the probable amount necessary for redemption of any unpaid warrants and the interest thereon, as well as the amount required for payment of interest and maturing principal on any outstanding bonds of the city issued for or in connection with any such electric light plant or distributing system. (2) The officer shall thereupon ascertain and prescribe as near as can conveniently be done an electric current rate or rates for the ensuing year which will create a fund sufficient to meet all requirements in subsection (1) of this section. (3) The officer may also include a further amount sufficient to create such fund, as in the judgment of the officer may be desirable or necessary to meet requirements of future contemplated additions, improvements or extensions to the plant or system.