Daily S3/EBS Storage costs are higher/lower month to month when storage usage remains static

Products

• CloudHealth

Issue/Introduction

The increase is tied to how AWS charges for S3/EBS Storage.

AWS charges using a unit of GB-Month which is your average storage over the course of a month and this figure is not fully finalized until the end of the month. 

For S3 billing details, please see:  https://aws.amazon.com/s3/faqs/#as10

Resolution

Using February as an example, compared to January it has approximately 10% fewer hours in the month (672 vs 744).

At the start of a month, in general, S3 storage costs are generally higher due to pricing tiers in place with S3 costs where the first 50TB/Month is charged at a higher rate and so on.

Here are details regarding AWS pricing:  https://aws.amazon.com/s3/pricing/

As the byte-hours increase, you start to hit the cheaper tier pricing which reduces the cost.

In addition, as February has only 28 days in a month, when AWS converts the Byte-Hours to the GB-Month to calculate the costs we see a higher daily cost as there are fewer days/hours in the month of Feb.

The final result should be in line with January (if storage remained static) as the total number of byte-hours will be lower due to the fewer hours in February.