Thank you again for all your answer, I find very useful to listen to different points of views.
Datasheets of electronic components have become my favorite reading recently, but this doesn't mean that I have understood well all I have read
On the PCB there is an Arduino Nano, an RTC DS3231 module, a TIP120, an L7808 and a few components and connectors (to 3 buttons, to an OLED screen, to the PS and to the solenoid valve.).
I will realize two versions of the project, one with 12V2A PS and one with a 24V2A PS.
24V is the only way I know to output 50W from a space of 8.5x5.8cm. I do really want to use a power supply already assembled and enclosed.
The system will be up and running for a maximum of 20 consecutive minutes (normally 5 are enough) and then it will have hours to cool down before the next watering.
The "brain" of the system (Nano+RTC+Screen) should not drain more than 150mW, so the L7808 must dissipate (24-8)*0.15=2.4Watt. Is it correct? Can a small passive cooler easily dissipate this amount?
I will immediately dig the suggestion from wvmarle of using a buck converter. I didn't know about this big difference with efficiency.
Regarding the stability of components plugged on Dupont sockets: Nano and the RTC are very firm, the TO220 not so much but they seem to be quite stable for the scope. Of course, I cannot play football with the box
I will proceed with the Duponts in the first version because I wish to replace easily the components.
Hi Grumpy_Mike, regarding the larger polarised capacitor on the output, on the L7800
SERIES specs I cannot find this need. Is it really needed, can the interferences affect the Arduino computation capabilities?