If the governing body decides to furnish electricity or water outside the municipal limits, it shall be done by a contract authorized by the governing body and executed on its part by the executive officer and the city auditor and by the customer or customers to be supplied. No such contract shall be authorized or entered into at any rate or price for electricity or water which shall discriminate against the inhabitants of the municipality, or which shall impose any direct tax burden upon the taxable property in the municipality, or in such amount as will interfere with the ability of the municipality to provide adequate electricity or water for its own use and the use of the inhabitants thereof.