During billing, running DB instance usage is first compared to your active Reserved Instances to minimize costs. Each hour, if the amount of running instances is less than or equal to the total Reserved Instances you have purchased, all running DB instances will be charged at the Reserved Instance rate. Any usage of running DB instances that exceeds the amount of applicable Reserved Instances you have purchased will be charged the On-Demand rate.

For example, if you own 3 Reserved Instances with the same database engine and instance type (or instance family, if size flexibility applies) in a given region, the billing system checks each hour to see how many total instances you have running that match those parameters. If it is 3 or less, you will be charged the Reserved Instance rate for each instance running that hour. If more than 3 are running, you will be charged the On-Demand rate for the additional instances.

If you purchase a Reserved Instance in a given region where you have a applicable running DB instance, the benefit will automatically be applied to that instance moving forward.