I have not been successful in getting the fill handle to fill only every other row, but I thought of some work arounds which I did not get to yet.
For the fill handle, fill in the first 4 instances yourself, and then excel will automatically understand what you mean when you double-click it to fill down.
Or you can wrap your function in IF(MOD(celladdress,2)=0,restofformula,"") to return blank if the cell row # is odd (or adjust it for even rows)
Just enter the formula once, select formula and one blank cell, and then fill down, it will only fill every other row.
2) separate date/time value and kwh into 2 columns, sum every second row (use fill handle), copy paste summed values (text only- not formulas) back into original kwh column, sort so that data with empty columns can be easily deleted.
I was going to suggest something similar but (I believe) a little easier. If you are doing this regularly I suggest using keyboard shortcuts and with my method you will not need to switch selected range once you fill down the formula so it should be quicker. Once you get used to this shortcut sequence it will be really quick.
Sum every second row (make sure to put formula in x:00 rows,and if you want 1:30 with 1:00 make sure to sum the proper rows, etc..). Just enter the formula once, select formula and one blank cell, and then fill down, it will only fill every other row.
Then with the cells still selected you can do the following:
Ctrl+C
Left Arrow
Alt+H, V, V (Paste Values in original column)
Alt+H, FD, S, K, Enter (Select blank cells from current selection [if you are doing this without shortcuts, it's Find & Select, Go to Special, Blanks])
Alt+H, D, R, (Delete rows of selected cells)
Right Arrow
Alt+H, D,C (Delete formula column) [Ctrl+Space, Ctrl+Minus is probably better for this step]