How big of an A/C for sealed room?

12x12 room

12 hydroponic buckets w/ 55 gal res.

2 600 watt hps

So how big of a unit to get temp to 78 degrees?

i do a 9x9x8 room and use a portable 12000 btu. works great till it starts to get 90+Fahrenheit.

Since the lights only cover 3x6 effectively, you'll be way ahead of the game if you cut down the room size to about 4x8. Nothing permanent is necessary, just some 2x2's and some panda plastic. Do this and a small 12k portable would be overkill even in summer. Seal it well and co2 injection would be easy. Place the fan on top of the filter and set it over in the corner to recycle and scrub the air and I see two lbs. of meganug in your future.
Fred

You'll be fine with as little as 6000 BTU, 8000-10000 would be on the safe side if you want to add another light.

I used a 6000btu Window Unit all summer with 2 600 Watts in a 8x10 room.

I would go with a 1.1 giggawatt unit. :0

I'd go with a 12,000 Btu mini split. Its oversized for what you need, but that is better than undersized, plus it has expansion capabilities also. Say you want to add 1- 1000w HPS light to your room or a few flouro panels for seedlings or clones.... This AC would allow that and still have the power to cool your space. Always go oversized on the AC, you don't want to spend a few thousand buying and installing it only to realize you should have spent the extra $500 to get the one size up because this unit isn't cutting it.

X

Mini splits are great, thats what i used, but a 12k btu minisplit can handle 4 600's efficiently.

Using too large an ac can be innefficient, you want it to use the ac properly.

You're both right - you don't want to skimp on AC because too small is the worst, but getting one that's too large will cause short cycles and prematurely wear out your unit.

The BTUs needed depend on what type of AC you get - I got a 14KBTU portable to handle four bare bulbed 600s and it couldn't keep my room (fully sealed, 14x10) under 84F with my CO2 generator running, so I needed to get a custom vertical cooltube.

Mini-splits are way more efficient if you're in a legal state or know a cool HVAC guy.

FYI, a good rule of thumb is 4KBTUs to every 1K in lighting (strictly a rule of thumb, your mileage will vary based on many different factors).

1200w shouldn't pump out that much heat to need an AC unit. Wouldn't it just be cheaper to air cool your lights?