The Robertshaw brand ST1301 Solar Thermostat is currently the only thermostat manufactured and specified for Solar Hot Water Heaters.
This Solar Thermostat has an auto-reset Over-Temperature Cut-out (OTC) for high yield sunshine days, taking the boost heater out of circuit. It's Auto Reset so you don't have to get up on the roof to turn it back on! The OTC protects the system against no-fault over temperature tripping. When the water temperature in the tank reduces to a normal operating level, the auto reset occurs and the heater can once again use the booster heater.
The ST1301 has a unique internal fuse, protection that is designed especially for Solar Water Heaters. This internal fuse is extra protection against any electrical malfunction , but is a one use fuse. If the fuse activates, the thermostat will need to be replaced.