Any electric distribution company that has a tariff for residential electric space heating customers shall maintain such tariff for a period of not less than five years after July 1, 2007. Such tariff shall be available for requests for electric service at a service location that was previously assigned to said tariff. Such tariff shall be available only to residential electric customers who use electric energy as the primary space heating source and who enter into an agreement with the electric distribution company for a period of not less than twelve months.