How can I determine why my A/C runs all day?
Your question almost exactly sums up the situation I had at my home last August.

What I found then was that despite a properly sized air conditioning unit and what I believe to be good insulation (the house holds heat well during the winter), the unit still couldn't keep up.

I don't have full proof, but I think a few factors caused my situation and might be worth checking for you:


The condenser (outdoor unit) sits in an "inside corner" with the house on two sides of it and a chimney and small storage shed that partially blocks a third side of it. This prevents good airflow across the unit. [I can't do anything about this without relocating the unit, which is not a DIY job for me.]
We had very little wind at all during the heat wave, which also added to the lack of airflow across the condenser.
The condenser was visibly dirty, which makes heat transfer to the outdoor air less efficient. Lots of dirt, dust, pollen, grass clippings, etc. build up on the fins over time. Normally rain will clean it off somewhat, but we hadn't had rain in ages.


Despite some advice I've seen not to do it, I used a garden hose to wash off the fins of the condenser (it gets wet from rain, right?) very gently, taking care not to make it full of mud or to bend the fins.

I could see water evaporating immediately on contact, and I have an infrared thermometer I used that showed a 30F or so drop in temperature in short order. The AC performed much better after that, but we also started to get a gentle breeze that helped things along.