I installed the fan last weekend. I bought a small one and it definitely cools, it was 15 degrees cooler outside, I ran it for 40 minutes and it cooled the house 6 degrees. It took 25 minutes to cool a whole degree, then rapidly cooled off the house for a total of 6 degrees. It seems that it takes so long to start the cooling due to built up heat in the house. After turning the fan off, it immediately rose 2 degrees , as again I believe it is latent heat in the house walls and furnishings. The total was 4 degrees in 40 minutes at a cost of about 2 cents in electricity. On a spring day in the mid 70s we did not have to turn on the A/C, as we would have had to do for about an hour or 2 due to radiant heat from the sun (newer house, no shade). Bear in mind, this is a very small 1450 cubic feet per minute fan, For max benefit I will need to install a second one down the hall. We only receive one rebate from the utility a year, so I will do this next year. My house is 2500 square feet and this only exchanges the air about every 20 minutes and my attic every 10 minutes ( according to my math, feel free to check it) below recommendations. Even at this, I believe we will decrease A/C use 1-3 hours per day, as we nearly always have cool mornings and hot days with low humidity. Peak A/C cost $2/hr