Power Vents
With balanced motor and blade design, power vents quickly exhaust heat or humidity from any attic. They work on a thermostat, which automatically turns the fan on when cooling is needed. Some models also have an extra feature called a humidistat that monitors the moisture level of the air.

Benefits:
One power attic ventilator is usually sufficient, unless it's a very large attic
Roof-mount power vents are ideal for hip roofs
Feature a preset adjustable thermostat for maximum homeowner convenience
Gable-mounted power vents are an excellent option for tile roof ventilation

Design Considerations:
Most power attic ventilators only have a thermostat, so they only monitor the heat in the attic. Moisture buildup is also a problem in the attic. A solution is a power attic ventilator featuring a combination thermostat/humidistat.
For proper attic ventilation, the Home Ventilation Institute recommends a system that provides at least 10 air exchanges an hour. To determine the fan capacity needed to provide this minimum airflow, use the following formula: attic square feet x 0.7 = CFM (cubic feet of air moved per minute).
Power vents are rated by CFM. Look for a power vent that provides at least the minimum CFM requirement. Remember, the higher the CFM the more air exchanges per hour.
Note: for roofs 7/12 or higher, you may want to add 20% more CFM capacity to handle the larger volume of attic space; 30% more for roofs 11/12 and higher
Keep in mind that there are energy costs associated with operating a power attic ventilator
Adequate soffit venting must be installed for best performance
If using gable vents, gable vents must be installed on the down-wind end of the home