That means the fan won't work at night or on cloudy days. If you need a fan to control moisture in the attic, you're better off with an electric (wired) fan that will run all the time. Wired fans are ...

Some have electric vent fans, and one house has a fan powered by ... Ice backs up under the roofing, then melts into the attic, causing water damage to insulation and the ceilings or walls below.