Its nice to have a switch disconnecting power to the solenoid like mentioned. I always test my weep when I know the temps are going to drop below freezing. It makes it easy to test when temps are above 36°. I had a weep gun malfunction once and didn't discover it untill the bay froze up. Then I installed the switch!
A 24V solenoid pulls a lot of amps. One Dema NO solenoid is .4 amps at 24V.
I had to research it - it wasn't as direct as I thought.
It goes vA ÷ V to get the amps the transformer can produce, so 40 vA ÷ 24 = 1.67A. Watts ÷ volts = amps, so 10 watts per solenoid ÷ 24 volts = .41 amps. That's assuming Dema NO solenoids - I've seen some, like ASCO, up to 22 watts.