We Germans like the most efficient solution.
I have a newsflash for you . The LM7809 has a MAX input voltage of 18V, NOT 24v.
The OP clearly stated that his input voltage is 24V. If your German you know that's > 18V.
And just for the record, the LM7809 is not the most efficient because the arduino doesn't need anything higher than 7V. if the OP had a lower input voltage , (like 18V) I would NOT have recommended the LM7809. I would have recommended the LM7808. If there was an LM7807 (like this one, I would recommend that. The arduino onboard 5V regulator is an LDO so 7V would be ideal, not 9V.
So there.