static head on drainback systems, where the solar fluid exits the loop and needs to be pumped back in when the system starts.

To calculate static head, measure the straight vertical distance between the highest point in the system (usually the top of the highest collector) to the level at which the fluid lies at rest (usually the top of the drainback tank). Add three or four feet as a safety margin. Note that the distance is not from the pump, which is usually below the resting water level. This is why drainback tanks are situated at the highest practical point below the collectors, where static head is minimized!

Static head = Distance from the highest system point to the resting water level + 4 feet

It is absolutely vital that your pump is powerful enough to overcome static head in a drainback system. If it cannot, some solar fluid will enter the system but will not complete the circuit. It will remain in the collectors and boil, potentially causing catastrophic damage.