Amazon EC2 Reserved Instances and Other AWS Service Reservation Models
AWS Whitepaper

Usage Billing

Except for DynamoDB reservations, which are billed based on throughput, reservations are billed for every clock-hour during the term you select, regardless of whether an instance is running or not. A clock-hour is defined as the standard 24-hour clock that runs from midnight to midnight and is divided into 24 hours (for example, 1:00:00 to 1:59:59 is one clock-hour).

You can find out about the charges and fees to your account by viewing the AWS Billing and Cost Management console. You can also examine your utilization and coverage, and receive reservation purchase recommendations, via AWS Cost Explorer. You can dive deeper into your reservations and Reserved Instance discount allocation via the AWS Cost and Usage Report.

For more information on Reserved Instance usage billing, see Usage Billing.