Oceanside has agreed to join three other North County cities sharing the costs of a study to determine the feasibility of forming a renewable-energy alternative to San Diego Gas & Electric Co. The study, expected to be completed by mid-summer, will look at the possibility of creating a nonprofit joint-powers authority to purchase electricity at a lower cost and higher percentage of renewable sources than that provided by SDG&E. The program is called a Community Choice Aggregation, or community choice energy.