Produce monthly dataset by calculation

cocomy

You can create a Calculated Table

From the Modelling Tab>>New Table

New Table =
ADDCOLUMNS (
    ADDCOLUMNS (
        GENERATE (
            SELECTCOLUMNS ( Table1, "Country", [Country] ),
            GENERATESERIES ( 1, 16 )
        ),
        "Date", EOMONTH ( DATE ( 2017, 1, 1 ), [Value] - 2 )
            + 1
    ),
    "Capacity",
    VAR Mycalc =
        CALCULATE (
            SUM ( Table1[Capacity] ),
            FILTER (
                Table1,
                Table1[Country] = EARLIER ( [Country] )
                    && Table1[Start] <= [Date]
            )
        )
    RETURN
        IF ( ISBLANK ( Mycalc ), 0, mycalc )
)

cocomy

Generateseries function

"Returns a single column table containing the values of an arithmetic series, that is, a sequence of values in which each differs from the preceding by a constant quantity. The name of the column returned is Value."

Since you needed 16 months (Jan 17 to April 18), I used GenerateSeries(1,16) to generate 16 rows

Then these were Crossjoined to each row of your existing table