... It doesn't seem to work, if 1 of the cells in the array is
blank... is there a way of putting in a condition where it ignores the
blank
cells?

Try this variant, array-enter (press CTRL+SHIFT+ENTER):
=SUM(IF(NOT(ISBLANK(A1:D1)),IF(LEFT(A1:D1,1)="S",R IGHT(A1:D1,LEN(A1:D1)-1)+0)))

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Max
Singapore