If the guy goes out to his truck and comes back in 15 minutes saying "yep, it's the right size" don't be surprised. He listened to the radio and played solitaire on his phone. The folks I called to my house to install a furnace simply wouldn't do the thorough calculation, they wanted to use the data from the old furnace or their "rule of thumb" (based only on the square footage of the house) even though I asked them to do the calculation. Every company I called wanted to install equipment that was too big.
Here's a good online calculator that replicates the Manual J computations.
Space-Pak calculator.
Notes:
- For "Cooling Outdoor Temperature" use 95 degrees. The recommended cooling design temp for LA is 90 degrees, but from what I recall there are enough days warmer than that that 95 is probably a safer bet.
- For "Number of rooms" count significant hallways that have any external walls (walls that abut the outside" as a "room"
- For each room, disregard the "people" entry unless there will be a lot of people there at one time (a conference room, etc.
- If you don't have a system that deliberately ventilates the house with fresh air from outside (most homes don't have such a thing), then pick "Infiltration BTU/HR SQ FT Exposed wall" for "Makeup Air Conditions"
- "Exposed Walls" are the walls in a room that are exposed to the outside. Don't include walls that have a heated/cooled space on the other side.
- Doors and windows: Enter data only for those that are exposed to the outside (i.e. not a door that leads to a hallway or another room). ALso, it says "Window areas are optional", but you should be sure to enter them, and pick the type of window that is closest to the type you have. Window area should be entered in sq feet.
- You don't need to be exact on any of this stuff, you just want to know if your AC is way too big. So, make a guess if you aren't sure of a particular entry, it will probably be close enough.
- When they ask for "location of the unit" they mean the condenser coil for the AC unit. That's the part of the AC system that is inside your house (in California they usually put them in the attic or in a small utility room/closet).
If you (or your contractor) don't like the calculator above, you may prefer
this one. It doesn't go room-by-room, but instead the data for each exterior wall of the home is entered on a single page.
Good luck.