SQL select, sum and group

I'm struggling with an Access database: I have a large database with the headers TAG, ZST, KL and R1H00 to R1H23 (24 of them, each one stands for one hour of a day) and I need to get a specific dataset out of it:

SELECT TAG, ZST, (R1H00 + R1H01 + R1H02 + R1H03 + R1H04 + R1H05 + R1H06 + R1H07 + R1H08 + R1H09 + R1H10 + R1H11 + R1H12 + R1H13 + R1H14 + R1H15 + R1H16 + R1H17 + R1H18 + R1H19 + R1H20 + R1H21 + R1H22 + R1H23) AS TOTAL
FROM Klassendaten
WHERE KL = "SWISS7_PW"

So far so good, but the result contains many items with the same ID (ZST). I need to sum all entries with the same ZST, but I couldn't manage to do it so far. (I tried to use the GROUP BY statement, but that only results in errors)

Any experienced SQL people here that could help me with this?