Sizing an air conditioner

I'm doing a garage conversion and haven't been able to find reliable numbers on the size of an AC unit to install. A local installer suggested a 2 ton minisplit system, and various online calculators suggest anything from 9,000 to 15,000 BTU's. I don't know who to believe.

Here's my situation: I live in Dallas, TX. My current garage is a detached 21' x 44' (two cars by two cars) structure. I'm installing a partition wall halfway back in the garage and am turning the back part into an air conditioned workshop. The workshop area is roughly 21' by 21'. There are no windows. The garage is concrete slab on grade. The three outside walls will be insulated with R11. The partition wall will have R11 insulation and an 8' wide insulated garage door that opens into the existing, unconditioned, garage space. I will insulate the attic space over the new workshop. The exterior walls are brick veneer. The three outside walls are not shaded. The workshop will be on the north side of the garage, so the south wall of the workshop faces the unconditioned garage space. The workshop will be occupied intermittently and usually be only one person. I will occasionally use power tools, a 3D printer, a laser cutter, or other appliances that generate heat.

How do I come up with a reasonable value for the size of an air conditioner to cool this space? I'm leaning toward a ductless minisplit system but could conceivably be convinced to use another type of system. The HOA doesn't allow window units, but if that's the best way to go I can probably make sure they don't know it's there.

Any recommendations the community has would be greatly appreciated.

 

Not an HVAC guy, but:
IIRC 1 ton of cooling is 12,000 btu's. Sounds to me like a 1.5 or 2 ton would be about right based on your calculations. I like a lot of cooling, and you are in TX so a bit extra might be a good thing.

I suspect you need to do a heat gain calculation. Here is one that seams to have some good info: Heating and cooling load calculators

The pros should be here later.

 

performing a manual J load calculation is the ONLY way to properly size an AC. Guessing does not cut it if you want proper performance.

 

That is an area of only 441-sf; one ton ought to easily handle the cooling in your area. However, do all the heat gain calcs you can...one ton might be too big.

Mini-splits are efficient so anything from 9000 to 10500-Btuh might handle the heatload; do not oversize the system...

 

Dallas TX summer 2.5% design is 100-F dry bulb & 75-wet bulb. Figured attic at R-30.(?)

I ran the available data you gave using an Excel spread sheet; with a window unit or ductless mini-split system, a ton unit should be more than enough for 75-F @ 50% relative humidity.

This is a raw estimate, all the data needed was not available, so you use it at your own risk. I am not liable for anything connected to these numbers & your choice of Btuh to cool that area.

Personally I believe 8,000 to 10,500-Btuh would cool it; also, you don't have to be out there on the extreme hottest day & hours.

I cool around 750-sf, with a lot of windows, with a 6,000-Btuh window unit & a large fan to circulate the air through the rooms & back to the window A/C.

We have some extreme Heat Index temps & it handles it A-Okay.

Remember you don't have to be out there on those extreme hours of exceptional heat-gain days.