(a) In determining rates charged by a municipal plant, the Public Utility Commission shall allow, in addition to all other factors, a reasonable rate of return on capital investments. The return shall be commensurate with that permitted private utilities having corresponding risks and equivalent to that necessary for private utilities to ensure confidence in the financial integrity of the enterprise so as to maintain its credit and attract new capital.
(b) Revenue received as a return on capital investment shall be retained by the municipal utility and held in a contingent fund for use by it in that or any subsequent fiscal year.