The greenhouse power should not be turned off during the summer months. 
The exhaust fans need to continue to operate. 
To save on energy cost, recommended by greenhouse engineers, turn to the highest setting for the thermostat or controls to be 100 degrees. 
The cooling system can be turned off either by adjusting that thermostat or unplugging the pump. 
The circulation fans can be turned off.

Anything over 120 or 125 can damage/destroy the environmental control system (most importantly).
The cost of this to replace is $400 for materials plus labor. 
The high temperatures will also shorten the life of the polycarbonate covering.

If the shut down is unavoidable open all vents and doors and make sure shade cloth is on knowing that you there are the above risks.
Not every time will the high temperatures damage the thermostats, but it will happen at some point sooner or later. 
Once power is restored all equipment should function as before without incident generally.

Dr. Teri Hamlin
North Region Agriculture Education
Georgia Department of Education
204C Four Towers University of Georgia
Athens, Ga 30602
706-542-3679 / 706-540-0032
[log in to unmask]