@pb: the amount of current 12VDC solenoid typically use depends on the coil resistance. I don’t have a 12VDC solenoid at hand so I don’t have enough knowledge to tell but I assume it would be similar to the typical current draw of a 24VAC solenoid, which is somewhere between 180 to 300mA.
Going back to your earlier questions:
1. long distance: the typical belief that higher voltage is better for long distance transfer is because when transmitting the same amount of power, higher voltage means lower current, hence the power loss under the same wire resistance is lower. But OpenSprinkler DC is designed to match the holding current of a typical 24VAC valve under 24VAC power, so this is not an issue.
2. +20V pulse: you can set the option ‘Boost Time’ to 0, then the controller will not boost the voltage any more.
So I think the key is to find the datasheet / spec of the 12VDC solenoid, which can help you figure out the current consumption and make the final decision.