i agree the first one is not great, the sum ifs is the way forward as that sums the column based on the end date.

Look at the formula in column F

I guess you type in the end date of the quarter in column C when the quarter ends (over writing the today() formula)? so E33 will sum everything in the column on sheet 2 upto the date in C33, you then drag it down to the next row at the start of the next quarter

This means you not have to add up the numbers but also you have a record of the daily amounts of sun if you needed to review or wanted to do further analysis