(a) The commission shall disallow, for purposes of setting the rates to be charged by any electrical, gas, or heat corporation for the services or commodities furnished by it, all expenses for advertising which encourage increased consumption of such services or commodities.
(b) Notwithstanding subdivision (a), the commission may allow, for purposes of setting rates, expenses for advertising which encourages the more efficient operation of the electric, gas, or heating plant, or for advertising which encourages the more efficient use of electricity, gas, or heat or the conservation of energy or natural resources, or presents accurate information on the economical purchase, maintenance, or effective use of electrical or gas appliances and devices.