How much Dynamo DB reserved capacity to buy
AWS charges you for DynamoDB based on usage, i.e.: how much data are stored, how much writes and reads are performed an so on.
Our application writes a lot of data so we pay for Dynamo mainly based on the number of Write Request Units we execute. Here below you can see the price AWS charges per million Request Units.
So, we're planning to buy some reserved capacity, to save on those prices.
The problem I'm having is to decide how many Provisioned Capacity Unit to purchase.
From Cost Explorer we know that WriteCapacityUnit-Hrs is the main cost driver.
How can I decide how much Dynamo DB reserved capacity to buy?
You should be very careful here, your images show you are using on-demand capacity mode tables, which you are unable to purchase reservations for.
Reserved capacity prices cover provisioned capacity costs only. Storage and on-demand capacity are billed separately. src
If you are in on-demand capacity mode, then an easy way to reduce your write throughput costs is to first move to provisioned capacity mode.
To evaluate how much capacity you should provision (not the same as reserved capacity) you can use this capacity mode evaluator.
Should you move to provisioned capacity or perhaps already are, and you wish to purchase reserved capacity, you can use this recommendation tool
And finally, if you happen to have access to your AWS account team, they can assist you with determining these values and help you to optimize.