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Two to three is a good estimate, but to determine the actual requirement, cooling load calculations would have to be performed. These calculations take into consideration: orientation of the house on the site, square footage of glass vs. square footage of framing, insulation thickness and quality, tightness of house, southern exposure, shading from trees, etc. House square footage is only a part of what is needed to do the calculations.
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Best of Luck, Brian
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