I would think that lack of fan during night time hours would only matter during periods of no wind, and then the only difference you would see would be a slight time lag.
While we are on the subject, the manual states that you can run the setup with 2, 1 or NO batteries.
These configurations will cause the fan to run slow, medium or fast. (When two batts are present, a certain amount of solar juice goes into charging them instead of running the fan).
Apparently, Davis tech support has not read this part of the book because when I asked them if there was any way to make the fan spin faster, they didn't suggest this.
I am running with one battery which does seem to work better (more accurate) than two...