Excel provides a handy custom calculation for creating running totals. However, if you have year and month fields in your PivotTable and you set running totals for the month field, the running totals are reset at the beginning of each year. You have a few ways to work around the problem:
1) Create a separate YearMonth field in PowerPivot, if you don't already have one in your table containing dates. Use the YearMonth field in the PivotTable instead of the month and years fields. Select a cell in the Value column you want to use for running totals. In the PivotTable Tools tab on the Ribbon, choose Show Value As-->Running Total in and select YearMonth as the base field.
Note: After you drag the YearMonth field to the PivotTable Field Row or Column box, the field will not be sorted in chronological order in the PivotTable. You must use a custom sort order for new dates to be sorted automatically. See "How can I automatically sort my Month or YearMonth field chronologically in a PowerPivot PivotTable? Excel always sorts fields placed in the Row and Column areas alphabetically," under H-PivotTables
2) If your running total is time dependent, you can use DAX time intelligence functions. For example, if you have a sales table named InternetSales, which contains a SalesAmount field and a Date table which contains dates in a FullDateAlternateKey field, first create a calculated measure for the sum of SalesAmount e.g. [Total Sales]=SUM('InternetSales'[SalesAmount]). Then create another measure to calcuate the running totals for [Total Sales] as follows:
=IF([Total Sales]<>"",[Total Sales](DATESBETWEEN('Date'[FullDateAlternateKey],BLANK(),LASTDATE('Date'[FullDateAlternateKey])),ALL('InternetSales')))
The formula allows you to use separate year and month fields in the PivotTable, and calculates running total across all years and months. Again, you will need to custom order your month field.
3) If you want to calculate running totals without date dependency, you must first order the row or column field accordingly. Assume that you have a YearMonth column taken from a Date table (the actual field could be anything with sequential significance of the field items). Let's assume that you're using the same sales data as in (2). First calculate [Total Sales] and then use this formula:
=[Total Sales]('Date'[YearMonth]<=VALUES('Date'[YearMonth]),ALL(Date'[YearMonth]))