I inherited a workbook for energy usage in which the formula below was
entered weekly as each week's readings were entered. Rather than do this
each week I dragged the formulae down each column so all I would need to do
was enter the reading each week. Simple? Yes, I thought so too.

However, I changed the =SUM formulae to =IF(L29=" "," ",7*2.466*0.21294)
to prevent blank readings such as #VALUE showing when there are no entries
in the row.

My problem is, dragging the formula down the columns 5 to 57, 3 out of 6
columns - in the same rows, 34 to 46 - the last entry made in row 13 of that
column displays when the cell should be blank. All other cells behave as
expected - blank.

All the rows and columns are identical except, of course, the cell numbers
within the formulae.

Can anyone explain how this happens and if there is a way I can cure it?

TIA or any help.