Tex. Utilities Code § 163.083

(a) An agency may establish and maintain rates and charges for electric power and energy the agency delivers, transmits, or exchanges. The rates and charges must:

(1) be reasonable and in accordance with prudent utility practices;

(2) be based on periodic cost of service studies and subject to modification, unless such a basis for rates and charges is waived by the purchaser by contract; and

(3) be developed to recover the agency's cost of producing and transmitting the electric power and energy, as applicable, which cost must include the amortization of capital investment.

(b) Notwithstanding Subsection (a), this state reserves its power to regulate an agency's rates and charges for electric energy supplied by the agency's facilities.

(c) Until obligations issued under this chapter have been paid and discharged, with all interest on the obligations, interest on unpaid interest installments on the obligations, and other connected and incurred costs or expenses, this state pledges to and agrees with the purchasers and successive holders of the obligations that it will not:

(1) limit or alter the power of an agency to establish and collect rates and charges under this section sufficient to pay:

(A) necessary operational and maintenance expenses;

(B) interest and principal on obligations issued by the agency;

(C) sinking funds and reserve fund payments; and

(D) other charges necessary to fulfill the terms of any agreement; or

(2) take any action that will impair the rights or remedies of the holders of the obligations.