What is the 20 rule for air conditioning?

What is the 20 rule for air conditioning is general guideline that residential central AC can maintain indoor temperature no more than 20 degrees Fahrenheit cooler than outdoor temperature, meaning if outdoor temperature is 95 degrees, AC can maintain indoor at 75 degrees, and if outdoor is 105 degrees, indoor may only be 85 degrees, which is normal due to equipment sizing via Manual J load calculation that sizes AC for design temperature not extreme heat, typically 75 indoor at 95 outdoor for most US climates. The 20 degree rule exists because AC units sized correctly for energy efficiency and humidity control, not oversized for extreme 110 degree days, as oversized unit causes short cycling, poor humidity control at 60 percent versus 50 percent, and higher bills, so correctly sized 3-ton AC for 2000 sq ft will maintain 20 degree difference, not 30 degree difference. If your AC cannot maintain 20 degree difference, for example outdoor 90 and indoor 80 only 10 degree difference, indicates problems like low refrigerant charge due to leak costing 600 to 1,200 dollars to fix coil, dirty condenser coil blocked with dirt reducing efficiency by 10 to 20 percent, dirty filter causing high head pressure, or undersized unit. Some variable speed AC like Trane XV20i and Carrier Infinity 21 can maintain 22 to 25 degree difference due to higher capacity at high outdoor temps. The 20 degree rule also applies to heat pumps in cooling mode. Best practice is to keep blinds closed, add insulation R-38 attic, and set thermostat to 78 during extreme heat to help AC maintain 20 degree rule and save energy.