Steve, your expression for the format looks ok. However, this may be related to the culture which is used.
If culture is not specified in your expression, the default current culture is used.
You can take a look at the help documentation:
Pivot Engine > Functions > Text Functions
Resources > Numeric Format Strings
Resources > Culture Table
You can update your expression like this:
=FORMAT(SUM(Fields!Total_Current_Charges), “N0”, “en-US”)
or if you prefer to use custom numeric formatting:
=FORMAT(SUM(Fields!Total_Current_Charges), “##,#”, “en-US”)
Nevron Support Team