I guess it depends how often it is 100 degrees?

It doesn't break 100F very often, but upper 90's is common from mid-June till early September (I'm near the FL panhandle), my a/c seems to struggle even when it's 95F outside.

sounds like 2.5 tons is right size for your place. You should add some insulation in your attic if possible. My parents house never gets cooled down either. They are sized right, but they don't have insulation in the attic. They are old fashioned and don't want 'dust' - lol.

The holy grail of the manual J is true. But - how many assumptions are usually made, how many blower door tests are done? Are the u-values of the windows known or guessed at? Is the attic insulation all messed up? Did the builder forget to insulate a wall to the attic from a bonus room? Was the insulation done sloppily so has inadequate r-value?

I'd go with real world but the point about the current unit possibly not being up to snuff is a good one. How long have you lived in home? Has it always been this way?

You know that the easiest answer is just insulate the attic more. It most likely isn't adequate. Then insulate/seal the duct work - if you found a leak - you know there are more - just smaller. I've seen DIY foaming kits - cost about $500 for a tank that might do all your ducts. 30% tax credit....

If that doesn't work then maybe go with 3 tons. Do remember that undersizing a unit 14 years ago was rare and the vast majority were oversized.

alabama

I know in the South it was not unusual for AC/HP condensers to be undersized and furnaces to be oversized.

run a man J load calc yourself to know for certain.

extra insulation can help but remember AC and HPs are normally sized for the average high summer temperature not the extreme temps that we have experienced over the past weeks.

I would not want to be borderline for cooling in the South.

If you decide that a three ton is best for you, ductwork may need to be upgraded to handle the additional BTUs. And I would want a thermostat that has "dehumidify on demand" feature for any new system you purchase.

IMO

Here is a link that might be useful: Man J Software for Load Calc

First thing, have an energy audit done. Check with your power company. They often sponsor them or have a list of approved firms available. The audit will not be very expensive.

The audit will point out where you can save money by upgrading the house's envelope. It's far cheaper to add insulation and seal leaks than it is to install a bigger system. Pay once with envelope upgrades, or pay forever with a bigger system.

david_cary makes a good point about insulation. Insulation in walls can also settle and not cover as it once did. People working in attics may move insulation and not put it back etc.

I'm working on getting a thermal imaging camera to spot these problems.

Our utility co-op has an energy audit they do for free and I guess it is worth what you pay for it. One of my customers laughed afterwords stating the audit person just walked through and made suggestions and did more talking than inspecting- 10 minutes! LOL.

"...AC and HPs are normally sized for the average high summer temperature not the extreme temps that we have experienced over the past weeks."

ASHRAE has tables in their manuals that give temperature and humidity limits and how often they will be exceeded.

These are the design limits used when sizing the system.

If the weather is outside the design limits the system may not be able to achieve the design set point indoors.

Systems rarely match perfectly.

If the Manual J calls for a 2.25 ton system a decision often has to be made between 2 tons or 2.5 tons.

Often the larger is chosen giving the system slightly more capacity than needed.
It will not significantly effect humidity control when you are down to small margins.

Customers rarely complain that the system works at slightly over the outside design temperature.

Complaints come from not hitting the set-point on hot days.

Manual J (load calculation) is always the focus, but it is only one step in a very dynamic process to properly designing a rsidential HVAC system. What is almost ALWAYS over looked are the other steps...

Manual S (equipment selection)
Manual T (air distribution)
Manual D (duct design)

TIP: Before hiring an HVAC contractor I would make him prove to me that he at least owns all the manuals. (they will fill a 4" binder) It doesn't guarrantee he actually understands them, but at least he has made an attempt.

2.5T or 3.0T?

Using a "rule of thumb", 2.5T for 1250 sf sounds about right. I could easily be wrong. Keep in mind even if the system has never performed well on hot days doesn't mean it is undersized. The number of residential systems that are designed/installed incorrectly is amazing.

I would focus on system performance before jumping in and buying a new system only because the current one doesn't keep up.

Focus on these areas first.

AIR FLOW
The proper amount of air flow through a system is critical to performance/capacity. It will change the Total Capacity as well as the SHR (Sensible Heat Ratio), the percentage of sensible capacity to total capacity.

What effects air flow...
1. Improper duct/system design. Under sized return air ducts are very common.
2. Improper maintenance - dirty coils (inside and out). blower wheels, filters.
3. Filter upgrades that were not anticipated in the original design, another common problem.
4. Closing off too many supply registers.

DUCT LEAKAGE
You can not "see" if a duct system is "sealed" properly. It must be tested, duct blaster or blower door to know for sure. If the return air duct(s) are in the attic, a quick DIY test is on a warm day run the fan, measure the temp of the air as it enters the return air grill, then the temp of the air as it enters the furnace/air handler (you will need to drill a very small hole to insert a temp probe at the return air plenum). If you see more than a degree or two temp increase you have leakage in the return air side of the system, a very common problem. Sucking in even a small amount of 120-130 degree attic air will place an additional load on the system.

REFRIGERANT CHARGE
Should be checked by a qualified service tech.

If you decide to buy a new system, you may find this link helpful.

Here is a link that might be useful: ACCA Checklist