All charges, made for electrical energy for residential, commercial, and farm purposes by any person, firm, corporation, or municipality engaged in the sale of electrical energy in cities of the first class having a population of more than five thousand and less than twenty-five thousand inhabitants as determined by the most recent federal decennial census or the most recent revised certified count by the United States Bureau of the Census, cities of the second class, villages, and unincorporated areas in Nebraska, shall be based on the amount of such energy actually furnished by the kilowatt-hour meter, together with such demand as may be registered or indicated by a demand meter, or as may be contracted for, to such purchaser. Such person, firm, corporation, or municipality may provide for either a penalty on or a discount from the amount of any bill to promote prompt payment thereof under uniform rules and regulations governing such penalty or discount. A reasonable minimum charge may be collected from purchasers of electrical energy by any such person, firm, corporation, or municipality, even though the charge for the amount of electrical energy actually furnished by the kilowatt-hour to such purchaser or user does not equal such minimum charge for the designated period of service. The provisions of sections 70-407 to 70-409 shall not be construed to affect any contract or franchise in existence at the time of the passage and approval of this section.