It is all about the numbers. In New Orleans if you have a 2,400 square foot (20,000 cubic feet)  house you better have at least a 4 ton air conditioner. That will keep you cool except on the hottest days when you ought to be at the beach or your summer house in the mountains anyway.  Four tons is 48,000 BTU. If your unit is fairly new say 5 years, the efficiency is a SEER of 12-14 and EER is about 10 which means you'll be consuming 48,000/10 watts or about 5 kilowatts when the AC is running.

Mine runs about 70% of the time 24 hours per day during the summer (June-October) and about 20% of the time the other seven months. Make that 18 hours per day. Entergy charges $0.10 per kilowatt hour in the summer. Half of this is their base rate and half is fuel adjustment. So air conditioning costs me 18*5*$0.1 or roughly $9 per day when it's hot. During the hottest five months it costs about $250 per month to be cool.

By the way that means the newer super high efficiency AC units make sense in New Orleans. If you spend $6,000 on a SEER 19 unit vs $2,500 for a 10, it will take you $3,500/$125  or just 28 summer months to get your money back. With 7 summer months per year in New Orleans you are making money after 4 years on a unit that is guaranteed for at least ten years. After ten years you are $5,250 ahead on a $3,500 investment. That is equivalent to a 10% interest rate.

Why do you need 4 tons?  One ton is equal to 12,000 BTU per hour, or 3517 watts
or 3024 Kcal per hour

The term ton goes back to the early days of refrigeration when making ice was the goal. 12,000 BTU per hour was just enough to freeze 2,000 pounds of water in a day and make a ton of ice. (Latent heat of fusion for water 334 kJ.kg^-1  = 144 BTU per lb_mass )

Four (4 ) tons is about 48,000 BTU of cooling. I could make 4 tons of ice every day with this system. Why do I need that much power just to keep my house cool?

Air Conditioning moves heat out of your house. So how does that heat get in?

• Leakage lets the outside in - this is the major culprit. Leaking duct work deserves its own analysis. Open doors, uncaulked single pane windows add'em up and you get the equivalent of an 8 square foot hole in the average house. It's like living with a window open at all times. Watch out, if you seal your house too well you run into other problems. Lets put leakage aside for the moment and look at the other sources of heat input.
  
• 10,000 BTU per hour gets through my wall and ceiling insulation - R19 or R30 means what?  U the thermal conductance of a surface is the inverse of R value. It is measured in Btu per ft²*degree F (in the nonSI world). So your R19 insulation will maintain a 19F difference for 1BTU per square foot. My 2000 ft² attic with what was originally R25 but is now more like R10 is chewing up about 6,000 of those 48,000 BTUs I have available. My 2,000 ft² of walls with R13 gets 4,000 more.
  
• 10,000 BTU per hour goes to cool unvented appliances - light bulbs,  electronics, toasters, hair dryers, irons, etc. . This might be why your 20 year old house isn't as cool as it was. You have much more stuff plugged in. A coffee machine and a hair dryer combine to produce 3 kilowatts. My five computers are "always on" adding about 1,000 watts to the household. We won't even talk about my TV's.  Every unvented watt you bring into your home will require about 1 BTU of airconditioning to move out.
  
• 1,000 BTU per hour on average during the daytime is needed to cool the vented cooking and cleaning spaces. The oven and range is vented but the kitchen still heats up when in use. Same for the washer and dryer in the laundry and the hot water for showers and stuff in the bathrooms. This stuff is intense but used for only a few hours during the day and the venting helps. Lets add another 500 watts of 24 hour average cooling.
  
• 500 BTU per hour is consumed by people radiating body heat. An adult male in an airconditioned room radiates / convects about 100 watts. Big guys more, kids less, animals too.
  

That's only about 22,000 BTU per hour accounted for excluding the leaks. Wow, is leakage really that big? Assuming the AC runs 70% of the time in the summer that means we're consuming about 33,000 BTU per hour. Even if I missed a ton of heat sources we are spending one third of our cooling bill on leaks. Could this possibly be right? Get out the caulk and door seals.