Effectiveness of Whole-House Fan

As I've written before, two years ago we installed a whole-house fan in an attempt to make better use of the sometimes-colder outdoor air to cool the inside of our house.  The fan has been great but I've been wondering if it was a good economical choice.  Would we have been better off just spending the money on air conditioning rather than this fancy fan?

To answer this question I would need to compare how long the air conditioner runs on days when we use it versus days when we don't.  Oh, and the days would have to be identical. Since I don't control the weather, doing a direct comparison is pretty much not going to happen.  The next best strategy is to find days with similar weather, use the fan one day and not the other, and do this a bunch of times to average out all the actual differences in the weather.  This is a better strategy but has two problems:

1. The decision on whether to use the fan or not must be made without knowing the upcoming weather for the day.  Obviously weather forecasts solve this somewhat but it may be hard to decide if the predicted weather for the day is equal enough to the weather in some arbitrary previous day (where I've already collected your data) to make the comparison worthwhile.
2. By definition, there will be days when I could use the fan but choose not to (so I can collect data for the air-conditioner-only case).  I bought this big fancy fan and I don't get to use it? And I'm doing this so that I somewhat scientifically determine if using this big fancy fan I already bought is doing what it should?  

In light of these two complications it became obvious to me that to do this experiment well requires more commitment to good scientific principles than I can bear right now.  Instead, I did the smallest experiment possible: two days that are vaguely similar in weather one using the fan and one not, measuring the amount of time the air conditioner runs on each day.

This graph shows the outdoor temperature on the two days I ran this experiment. The days are far from identical but do have a similar daytime highs and general temperature trends throughout the day.

The measured air-conditioner runs times are as follows: fan + AC: 3.8 hrs, AC only: 5.1 hrs.  That's a time savings of 24% when running only the fan.  The real question is how much money this saves me and for that I need to know how much energy my air-conditioner uses.  I don't have a way of measuring that at the moment but I've got one in the works. For now I'm going to have to be satisfied with run-time.

UPDATE: I've got a partial follow-up where I get an estimate of the air conditioners power consumption.