Hey everyone! I've been exploring AWS Cost Explorer and I've run into some confusing parts, especially concerning amortized costs related to the recurring charge type. I'm working on creating an annual cost view from the last 40 days of data I pulled from the Cost Explorer API.
The issue arises when trying to annualize the costs for Reserved Instances (RIs) and Savings Plans (SPs), mainly because there are upfront fees that might not show up in those 40 days and monthly recurring fees for RIs that could land on random days within that window. Amortized costs are crucial since they distribute both the one-time upfront and monthly charges across the billing cycle.
According to the Cost Explorer documentation, for the daily view, any unused part of upfront reservation fees and recurring RI charges shows up on the first of the month. Here's what I get:
1. Under 'DiscountedUsage', the amortized cost corresponds to the effective usage of upfront costs and monthly fees over the billing period.
2. Under 'Recurring', we see any unused recurring fees and the amortized upfront fee, all on the first day of the month.
However, I'm trying to clarify what is meant by 'unused'. Does it mean:
a) Any purchased RI that isn't utilized by a corresponding instance at all?
b) For the mid-month situation, does it refer to the monthly fee and amortized upfront costs that haven't been utilized yet, meaning this unused amount would decrease as we utilize more throughout the month?
I'm leaning towards (a) being correct but I'm not entirely sure about (b). Let me know your thoughts!
3 Answers
In our experience, ‘unused’ in the Recurring charge type specifically refers to any portion of the monthly RI commitment that wasn’t utilized. For instance, if you have a monthly RI and only use it 70% of the time, the remaining 30% shows up as amortized cost on the 1st of the month. It’s not about timing—it's about actual usage. The upfront cost gets fully amortized over the month regardless of whether it’s used, and any unused recurring part doesn’t diminish over the month; it’s a fixed reflection of the gap for that month. We found a new tool called pointfive that simplifies all this, sorting out the amortization logic and clarifying what gets used and what remains unused, which really helped our reporting consistency.
From my understanding, it actually captures both components. It includes what hasn’t been used from your total purchased period and also what’s essentially wasted. If it’s not linked to a resource that's actively using it, it counts as unused.
From what I understand, it’s mostly (a) minus (b). The amortized view is pretty helpful for tracking RI and SP usage, and honestly, it handles everything fairly well on a daily basis. If it were the other way around, it would complicate things a lot more.
I totally agree! I’ve done some testing myself and it looks like it really is just (a). I’ll check back in a few days to see if anything changes.
I think it would make more sense if they spread that unused portion across hours or days instead of just lumping it on the first of the month. It can be pretty confusing as is.