(a) After January 31, 1992, no utility shall commence the construction of an electric plant, sell an existing plant or any portion thereof which is included in the retail rate base or which has been certified, enter into a long-term purchase of electric power, or make expenditures for a demand-side capacity option for serving the utility's Georgia retail customers without having first obtained from the commission a certificate that public convenience and necessity requires, or will require, such construction, sale, purchase, or expenditure.

(b) No utility shall increase or decrease the capacity of:

(1) A generating unit of an electric power plant;

(2) A long-term power purchase; or

(3) A demand-side capacity option

by more than 15 percent of its demonstrated capacity in megawatts for serving the utility's Georgia retail customers without first obtaining a certificate or an amendment to a certificate, as appropriate, that public convenience or necessity requires or will require such increase or decrease; provided, however, no certificate shall be required if the increase or decrease is caused by a rule, regulation, or law mandated by any duly constituted local, state, or federal governmental body or agency or is caused by power pooling, forced or maintenance outages, or short-term sales for a period of less than one year.