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

Conclusion

Effectively planned and managed, reservations can help you achieve significant discounts for AWS workloads that run on a predictable schedule. It’s important to analyze your current AWS usage to select the right reservation attributes from the start and to devise a longer-term strategy for monitoring and managing your Reserved Instances. Using tools such as the AWS Cost and Usage report, and the Reserved Instance Utilization and Coverage reports in AWS Cost Explorer, you can examine your overall usage and discover opportunities for greater cost efficiencies.