BOSTON – Eastern Bank plans to “green” all of its electricity use with renewable-energy certificates from Constellation Energy. The bank has signed an agreement to allow Constellation to supply more than 30 million kilowatt-hours of electricity to Eastern Bank’s 94 branch offices and five corporate centers for the next three years. The energy provided will be “greened” through Constellation renewable-energy certificates. The certificates are Green-e Energy Certified and sourced from wind energy. They give developers of renewable energy a market-based, financial incentive to build wind, solar and other forms of renewable energy. One certificate is created for each megawatt-hour of renewable power that is delivered to the electric grid.