Good morning @SMEchtler,
I appreciate you posting your question regarding credit consumption. When using ArcGIS for Excel, you are essentially using your ArcGIS Online organizational account. Charges will occur within ArcGIS Online even though the are 'working in Excel'.
For example, it costs less than 5 credits to geocode 125 addresses, store 2 GB of map tile data, or enrich ZIP Code boundaries in Detroit, Michigan, with four variables of population and income data. See also Understanding credits
My guess for charges per day is there maybe associated credit cost for storage (9 credits per day).
As for your questions:
Q. Do I get charged every time I open an Excel SS with this layer on the map, even if I did not add the layer to a tab?
A. You should not get charged every time you open an Excel spreadsheet but what you are charged for is dependent on what you are doing in the ArcGIS for Excel map.
Q. Also, why didn't it stop my at 50 credits, which is what I have my limit set at?
A. Unfortunately I would not be able to determine this without looking at how the credit budget was set for the organization or for specific members. From my understanding, a user is blocked if they will surpass the allocated credits allowed (in your case 50 credits). Credit budgeting applies to transaction-based services and tools such as GeoEnrichment and spatial analysis. It sets a threshold of credits that a user can spend in total or in a single job, preventing them from accidentally spending a large number of credits at once. Administrators can monitor credit usage through the status page which may be helpful. See View and report status.
Q. Has this happened to others?
A. I have not seen a reoccurring issue with regards to credits not stopping after a limit is set in ArcGIS for Excel.
If you feel any of the charges are in error I would encourage you to contact your Customer Service representative (1-888-377-4575). We have members of our staff who can review your account and in some situations refund discrepancies.
As for some workarounds going forward...I would recommend allocating credits by user (or credit budgeting) and also cleaning up the organizational account, deleting content that is not needed so storage cost is optimized and lastly, review what Credits by capability cost.
I hope this helps! I completely understand we do not have credits to just throw away. Also do not hesitate to reach out again if there is any further questions/concerns.
Best,
SC
Shannon Cardoza
Product Engineer
