The Solar Star Attic Fan by design is meant to run whenever daylight strikes the solar panel.  Attics are meant to be ventilated year-round. During the winter, proper ventilation helps prevent condensation from forming within the attic which could cause significant heat loss from the home if the attic insulation gets wet.  A cool, dry attic is the best solution for keeping the heat in the house during the winter.  If your attic is properly insulated and moisture build up is not a concern but cold temperatures are a concern then consider using the optional Solar Star Thermal Switch.
The Solar Star Attic Fan does not have an on/off switch but the optional Solar Star Thermal Switch can be added that will turn off the Solar Star Attic Fan when the temperature in the attic is below 65 F (18°C) and will turn on the Solar Star when the temperature in the attic is greater than 85°F (29°C).