One way:

=SUM(IF(ISERROR(A2:A100),0,A2:A100))

The formula is an array formula and must be
entered with <Shift<Ctrl<Enter, also if
edited later.

--
Best Regards
Leo Heuser

Followup to newsgroup only please.

"René" skrev i en meddelelse
...
I have a table where I lookup working hours in a "Incident reporting
sheet"
I want to sum these hours per week. Some of the cells are invalid (#N/A),
because there were no incidents that day. But then I can't get the sum for
those specific weeks. How do I ignore the invalid cells in summing those
arrays?