I'm looking to install a window A/C thru the sidewall of my shop. Its 24x36 (872 sq ft) with 10' sidewalls and is open to the roof. 4" sidewalls with insulation & sheetrock covered. The roof is insulated with 4" fiberglas bats - no sheetrock.

Lowes has a 25K BTU unit that is advertised to cool 1672 sq ft. Or a 18.5K BTU unit that will cool 1170 sq ft. Price difference is only $50.00

I will not be trying to get 72 degree temps. If I can get indoor temps 10-15 degrees cooler than outside I will be happy. Temperatures here are frequently 95-100 but we have very little humidity.

Will the 10' sidewalls and open ceiling area require the larger unit?

I'm looking to install a window A/C thru the sidewall of my shop. Its 24x36 (872 sq ft) with 10' sidewalls and is open to the roof. 4" sidewalls with insulation & sheetrock covered. The roof is insulated with 4" fiberglas bats - no sheetrock.

Lowes has a 25K BTU unit that is advertised to cool 1672 sq ft. Or a 18.5K BTU unit that will cool 1170 sq ft. Price difference is only $50.00

I will not be trying to get 72 degree temps. If I can get indoor temps 10-15 degrees cooler than outside I will be happy. Temperatures here are frequently 95-100 but we have very little humidity.

Will the 10' sidewalls and open ceiling area require the larger unit?

Same size Garage and same AC unit. I did add a 5k window unit at the opposite end of the Garage. Works great, but pulls a lot of juice. Cool is not free.

Air conditioners are sized for 8' ceiling heights with normal insulation. You have a lot more cubic feet to cool and not much insulation in your ceiling. I would beef up your insulation and use the smaller unit. You only pay for the insulation once. The electric bill comes every month. I can cool my 625 sq. ft. shop (9' ceiling) with a 5000 btu air conditioner but we're not as hot here and my ceiling is insulated to R30.

In Southern California, where it's not very humid, swamp coolers work great and draw very little electricity. All you have to run special is a tiny water line. You can plumb it into a faucet easily.

Plus you can fit a small swamp cooler in about the same size hole as a 'window type' ac unit.

Give it a try, we have one in our garage and it works great.

I have a 990 sq. foot garage; 8' walls sitting on a base of concrete blocks--about 9 foot altogether. Completely sheetrocked; 4" fiberglass insulation walls and blow-in insulation in the rafters.

I put in a 10K window unit the first summer after the garage was built; it did just what you want--knocked down the inside temperature, and de-humidified the air but did not produce "cold" interior temps. That was a miserable hot 'n' humid summer; the garage was too warm to be comfy for a house, but just fine for a garage. De-humidifying helps as much as the actual temperature drop.

Since then, it hasn't been warm enough to turn it on.

I agree with animal69: Buy the insulation first--including the ceiling/rafters; THEN buy the air conditioner.

Portable A/C units would probably be a good choice provided you can pop a series of closeable vent holes in the walls; you just roll the A/C unit close to where you're working and let it blow directly on you. I'd have probably done that except EVERY portable A/C unit I looked at was Chinese; while the Westinghouse window unit I bought was "Assembled in the USA" (from Chinese components, I found out later!)