This solenoid valve is very important to prevent the system from overflowing in case of a power outage.  It is powered by a 110 volt outlet, and only consumes about 10 watts.  When the power is on, the valve is open to free flow.  If the power fails the valve closes and no water can flow.  Solenoid valves come in many different specifications.  They can be either open or closed when power is applied, come in both 12 and 110 volt versions, but most importantly for my application is that it had to operate at 0 psi when open, as this was a simple gravity feed system.  Regular lawn irrigation solenoid valves are cheaper but require the higher pressure of your house water system to enable water flow.  I was able to find this valve at Grainger Industrial Supply available on the web at