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 FunctionsResources >
Numeric Format StringsResources >
Culture TableYou 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”)
Best Regards,
Nevron Support Team