Interesting thing to test it. From a clean 9V battery. I don't know how long can last such a battery, since the mega with eth shield draws a lot of current ( I guess around 300mA)...But it's worth testing it.
So, so what ? Do you actually think that 24V noise generated on one wall socket outlet WILL NOT affect something plugged into the OTHER socket on the SAME WALL PLATE ?
RC SNUBBER CIRCUITI was aware of the use of snubbers to eliminate relay contact arcing but don't see how they apply to a solenoid that has no contacts.Can you provide any math to support that ?