Billing methodology for EC2 Reserved Instances

Question

Hi,  
I've a basic query around how the billing is done for EC2 Reserved Instances (RI).  
Say, I buy RI for 1 instance only of type m4.large (for 1 year term).  
And I create 2 on-demand m4.large instances and use them for 1 year. However, they were run for 12 hours a day only (not full day / 24 hours).  
In that case, how the billing will happen?  
Will it charge me 1 year full RI rate for the 1st instance (even if it was not running continuously) + the on-demand rate for the 2nd instance (for 12 hours daily)?  
Or will it charge me just 1 year full RI rate for the 2 instance combined (since running 2 instance for 12 hours each is kind-of-equivalent-to running 1 instance for 24 hours -- at least in terms of total hours)?  
  
Any pointer to AWS documentation would be also helpful (I searched already, but it was not clear for a use case like the above).  
Thanks in advance.

Answer

Hello,  
Thank you for reaching out  
  
The RI only applies to one on-demand EC2 instance using an m4.large instance class. RI billing continues to count even when the instance is in a stop state. However, when using on-demand instance, you only pay for the number of hours the instance is in an active state.  
Please note that you will be charged 1 year full RI rate for the 1st instance and on-demand rate for the 2nd instance.   
  
Here is a documentation on Reserved Instances for more information:  
https://docs.amazonaws.cn/en_us/AWSEC2/latest/UserGuide/apply_ri.html  
You can refer to the link below for the simple monthly calculator:  
https://calculator.s3.amazonaws.com/index.html  
  
Thank you

Answer

Thank you. That helps.  
  
Edited by: sst on Oct 16, 2019 5:30 AM

Billing methodology for EC2 Reserved Instances

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