Public utilities purchasing electrical energy in accordance with the provisions of this chapter shall pay rates per kilowatt hour to be set from time to time by the commission. Such rates shall be based on the purchasing utility’s avoided costs. The commission may set long term rates which shall, at the option of the qualifying small power producer or qualifying cogenerator, be based on the purchasing utility’s avoided costs either calculated for the time of delivery or calculated for a specified term at the time the qualifying small power producer or qualifying cogenerator agrees to be obligated to deliver for the specified term. Nothing in this section shall limit the authority of any electric utility or any qualifying small power producer or qualifying cogenerator to agree to a rate for any purchase which differs from the rate or terms or conditions which would otherwise be required by the commission. No payments or rates shall be required by this section in locations where retail electric competition is certified to exist pursuant to N.H. Rev. Stat. § 38:36, unless such payments or rates are pursuant to an arrangement authorized by N.H. Rev. Stat. § 362-A:3.