Powered roof vents make noise, though the amount of fan noise that can be heard indoors is tempered by the home's insulation and building materials, and varies from house to house. Thermostats control the fan operation, usually operating the fan when the attic temperature is above 100 degrees. This can extend into the evening in warmer climates, resulting in fans that operate when the family is trying to sleep.
Passive vents are simple weatherproof grills or covers with no moving parts, whereas power vents include an electric motor and blower or fan assembly, producing a higher cost. The fans are heavier than passive vents and require more installation time. Special electrical lines to the attic insulation may also be required for power vents, adding to the installation cost.
Electric power vents consume about 300 watts of electricity while operating--though actual run time determines the exact cost of operation--but the proper ventilation of the attic should reduce air conditioning costs, ultimately saving you money. In addition, solar-powered power vents are available, while passive attic vents utilize no energy.
Attic power vents are mechanical devices with moving and electrical parts, so failure is possible. Passive attic vents have no moving or electrical parts, and thus have no potential for failure that can result in additional repair or replacement costs.