This excerpt taken from the EOC 20-F filed Jun 8, 2009.
Sales to Other Generation Companies
Each CDEC annually determines a firm capacity for each power plant. The generator is allowed to sell capacity up to its firm capacity. Firm capacity is the highest capacity which a generator may supply to the system at certain peak hours, taking into consideration statistical information regarding the time it is out of service for maintenance.
A generation company may be required to purchase or sell energy or capacity in the spot market at any time, depending upon its contractual requirements in relation to the amount of electricity to be dispatched from such company and to its firm capacity.