You get the best discount from your Reserved Instances when you run an EC2 instance with attributes that match the terms of your Reserved Instance. If you stop or terminate the EC2 instances that benefited from the discounted price of your Reserved Instance, then you won't get the full discount from your Reserved Instance.

To receive a notification when you're not getting the maximum discount from your Reserved Instances, use AWS Budgets.

Note: Before you begin, be sure you have the necessary permissions to access Cost Explorer and AWS Budgets.

1. Sign in to the Billing and Cost Management console and
   open the Budgets pane.
2. Choose Create Budget.
3. Under Budget details, for Budget Type, choose RI Utilization. For all other options, choose what best fits your use case.
4. Under Refine your budget, if your account is not part of an organization, select all of the options except Linked Account
   .
   If your account is the master account in an organization, select all the options.
5. Under Notifications, choose the following options:
   For Notification Criteria, type 90.
   Note: This should be your preferred RI utilization threshold, but you can adjust the percentage to fit your use case.
   For Email contacts, enter the email address(es) you want to receive a notification.
6. Choose Create.

If you use these options, you are notified by email whenever you're receiving less than 90% of the potential discount your Reserved Instances offer.
Note: Reserved Instance utilization data can take up to three days to update.