FILTERED SUM Over Many Rows

I would first create two new dimension tables: Accounts and Stores.

Since you're using SQL, it should be something akin to 

SELECT DISTINCT accountnumber AS "Account Number" FROM TABLE

SELECT DISTINCT store# AS "Store #" FROM TABLE

Then, create a relationship between these tables and the appropriate columns in the fact table above.

Next, as you suspected, I would reccomend transforming the fact table.

• Rename the colums: janbalance = 1, febbalance = 2, etc.
• Unpivot the monthly balance columns. We'll call their numbers "Month" and their value "Balance".
• Create a date foreign key for each row.

In powerQuery, you can do something like:

= Table.AddColumn(#"Changed Type", "Year-Month", each Text.Combine({Text.From([Month], "en-US"), "/1/", DateTime.ToText([Last day of the year], "yyyy")}), type date)

• ​I'd create a calendar table with month, year, year-month, etc;.

Then, connect it to the "Year-Month" column we've just created.

Now you're set!

SUM the Balance column, while using the accounts, stores and calendar tables we created (instead of the columns in the fact table).

I hope it works!