Why am I being billed after I bought the recommended number of EC2 reserved instances?

Question

I had an expiring RI for a t4g.large instance, and Amazon recommended I buy (8) t4g.nano RIs. I have since replaced the t4g.large instance with a t4g.medium and t4g.nano instance, thinking I was reducing my overall usage. But now the Cost Explorer is showing my costs have risen over 100%, the RI Utilization Report shows a steep decline, which matches the decline in the RI Coverage Report. Worse, the recommendation engine is now recommending I buy (10) more t4g.nano instances.

All my instances are running in the same Availability Zone (us-east-1c). They're all t4g instances. They all run Ubuntu. They all have default tenancy (shared). My usage has been steady for the last year. The only change I can see is the RI conversion.

I'm not getting the expected benefit of the nano RIs. What is going on? How do I get these new RIs to cover my usage?

According to the [AWS Docs](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/apply_ri.html#ri-instance-size-flexibility), 8 nano RIs wasn't enough to cover the large instance, but 18 nano RIs is way more than I need. And if I was maxing out the RIs, I'd expect the Utilization Report to stay steady at 100%.

Edit: added usage information

Accepted Answer

Thanks for the advice. It turns out the RIs were in the Availability Zone, not in the Region, so I was not getting the benefit of size-flexibility. The thing that eventually tipped me off was I bought 8 "nano" RIs, and had one nano instance running. My Utilization Report showed 12.5%, so I figured the RIs weren't being applied to my larger instances. I changed the RIs to be in the Region, and my Utilization Report immediately improved.

Answer

Please note that Cost Explorer assumes that your future usage is the same as your previous usage. So your recommendation highly depends on your past usage for the duration you chose. Hence , Cost Explorer assumes that your historical usage reflects your future usage when determining which RIs to recommend.
Can you check the cost associated with service, account and time line ?Has your recent activity/workload changed during this time? Is that anticipated ? 
Can yo cross verify your usage with Reservations utilization breakdown ?

Answer

As you may know, that all the recommendation is based on your usage and you can select the usage duration for last 7/30 or 60 days. Cost Explorer is your best option to review and understand the usage and coverage in order to review your recommendations which would help in a big way!
Another thing to note that there is a normalization factor that you need to consider which would match the recommendations. 
For instance size nano= 0.25 (normalization factor), micro=0.5, small=1, medium=2 and large=4, similarly the rest of them.
I would recommend to consider reviewing this [Link](https://docs.aws.amazon.com/whitepapers/latest/cost-optimization-reservation-models/maximizing-utilization-with-size-flexibility-in-regional-reserved-instances.html)

Why am I being billed after I bought the recommended number of EC2 reserved instances?

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