Why has an inactive EC2 instance exceeded 85% of the monthly usage limit?

Even though you haven't actively used your EC2 instance since mid-February, there are several reasons why you might have exceeded 85% of the monthly usage limit for AWS Free Tier-eligible services:

1. EBS Volume: The most likely cause of your continued usage is the Elastic Block Store (EBS) volume associated with your EC2 instance. EBS volumes continue to incur charges and count towards your Free Tier usage even when the EC2 instance is stopped. The 25.6 GB-Mo usage you're seeing is likely from the EBS volume that's still attached to your stopped instance.

2. Stopped vs. Terminated: If you only stopped your EC2 instance instead of terminating it, the associated resources like EBS volumes remain intact and continue to count towards your usage.

3. Free Tier Limits: The AWS Free Tier typically includes 30 GB of EBS storage per month. Your usage of 25.6 GB-Mo is approaching this limit, which explains the 85% usage warning.

4. Billing Period: AWS calculates usage on a monthly basis, starting from the date you created your AWS account. This period might not align with calendar months, so usage from late February could be counted in your March billing cycle.

To address this issue and prevent further unexpected usage:

1. Terminate (not just stop) your EC2 instance if you're not planning to use it soon.
2. Delete any unnecessary EBS volumes or snapshots.
3. Check for any other resources you might have created during your testing, such as Elastic IP addresses, and release them if not needed.
4. Regularly monitor your usage through the AWS Billing Dashboard or EC2 Dashboard to track your consumption and stay within the Free Tier limits.

For future reference, it's a good practice to terminate instances and delete associated resources when you're done testing to avoid unexpected charges or usage.
Sources
AWS Free Usage Tier (Global-DataTransfer-Regional-Bytes) | AWS re:Post
Billing misunderstand | AWS re:Post
Track your Free Tier usage for Amazon EC2 - Amazon Elastic Compute Cloud

This is EBS (Elastic Block Store, which is basically the virtual hard disk attached to your EC2 instance) and this is charged for the whole duration that it exists, regardless of whether the EC2 instance that it's attached to is running or stopped.

This is different to an EC2 instance, which only accrues charges (or consumes Free Tier entitlement) whenever it is running, and not when it is stopped.

Go to https://console.aws.amazon.com/billing/home#/freetier and it will show what your free tier entitlements are, and how much you have used of each.

Looking at what the figure 25.6048383 GB-Mo actually means - suppose you have a 30GB volume that exists for the entire month, that would be expressed as 30 GB-Mo, i.e. literally 30GB-per-month (the same arithmetic means that a 60GB volume that existed for half a month would also come out as 30GB/month and so on).

You posted your question on the 27th of March, which is approx 85% of the way through the month, and you've just had the alert that you've exceeded 85% of the usage limit. If you have a 30GB volume, and if this is the only volume in your account, then it will hit 100% of Free Tier usage at exactly the end of the month, so you should never actually be charged for it so long as your account remains in-scope of Free Tier, and there are no other volumes in your account.