Why can't your AC cope?
I know mine runs long but does make it down. My house is old and not well insulated.
I have an 1800ft2 mid-50's house with mediocre insulation in most places and with an inunsulated great room cathedral ceiling (ranch). When direct sun hits that roof, the heat just gets in. My rather new 3 ton variable speed MERV 13 Traine just can't keep up. It would probably do fine if not for the cathedral ceiling.
Unfortunately, there are already issues with the great room ceiling and it is only 6" rafters, the drywall is missing in some places from moving of the skylights and in poor condition elsewhere. The only fix is to completely remove the entire ceiling, thicken the rafters, add insulation in the now reasonable stud cavities, and all new drywall/painting. And the thickening means issues with the two wood-covered walls where they meet the ceiling. So it is a hugely major project that I just can't do while living in here, especially with two geriatric cats.
I should have put in a 4 ton unit (they didn't make 3.5 ton and said a 3 would suffice). Now I am stuck.
On top of all that, this house sits on an uninsulated slab and has no vapor barrier there or in any of the walls or ceiling. So I have almost no control over humidity. Even with the AC running on 100%, it is not uncommon for the humidity inside to be over 65%. And in the winter, it plummets to 10% inside. So I also have to run a dehumidifier half the year and humidifier the other half. Even with those, I am lucky to keep it 50% in summer and 30% in winter. Constant noise, more electricity, waste heat, filters, humidifier dust, etc, etc. Royal pain. I long for a house with real insulation and vapor control. And that isn't happening at this point.
<end of sob story>