EC2 Instance Savings Plans

EC2 Instance Savings Plans start applying to your eligible usage immediately after purchase, unlike RDS Reserved Instances which typically begin at the start of the next billing cycle.

When you purchase EC2 Instance Savings Plans, they are automatically applied to your eligible EC2 instance usage right away. The system continuously evaluates your usage and applies the Savings Plans rates to the appropriate instances within the committed family and region.

You should start seeing the cost differences reflected in your billing almost immediately. The Savings Plans will apply to your usage after any existing Reserved Instances are applied, as Reserved Instances take precedence.

It's important to note that while the savings start immediately, you may not see the full impact until you've had a chance to accumulate some usage under the new pricing. Additionally, the exact timing of when these savings appear in your cost reports or billing dashboard may vary slightly depending on when the usage data is processed and updated.

To get the most out of your EC2 Instance Savings Plans, ensure that you're running instances that match the family and region you've committed to. Remember, these plans offer flexibility in instance size, OS, and tenancy within the chosen family and region, automatically applying to the eligible usage to maximize your savings.

If you want to track the application of your Savings Plans more closely, you can use AWS Cost Explorer or set up AWS Budgets to monitor your spending and savings over time.

Sources
Compute Savings Plans and Reserved Instances - Savings Plans
Main Differences b/t EC2 Saving Plans (Compute and Instance) and Reserved Instances | AWS re:Post
Understanding how Savings Plans apply to your usage - Savings Plans
Savings Plans - Amazon EC2 Reserved Instances and Other AWS Reservation Models