DAX: How to perform a cummulative summation

First you need to have a seperate date table in order to perfom date calculations...

Step 1 : Create a date table -> Go to Modelling click New Table -> enter   Dates = CALENDARAUTO()..Now you have a date table call Dates..-> New Column in this table  Month = MONTH(Dates[Date])

Step 2 : Create Relantionship between your   yourtable[Dates] ( the new only dates you created ) and Dates[Date] ( the calculated table)

Step 3 : Rewrite your formula to Measure = CALCULATE(
                         SUM(rprtsolarhistorical[Daily Output]);
                         FILTER(
                                         ALL(Dates[Dates]);
                                        Dates[Date]) <= MAX(Dates[Date])
                         )
                   )

Step 4: add the months field from the new Dates table and the "measure"

Hope this works