I'm building a 16'x20' storage building with a barn roof, so it's pretty much a 2 story area. Will I calculate the BTU's using 320 sq.ft. for my window unit? (Not for heating, just cooling.)
Are BTU’s only calculated using square feet
air-conditioning
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It's a vent for a portable air conditioning unit, and stop looking in my windows!
The other side of it would look like this:
The EER (energy efficiency ratio) is a lovely mashup of units (BTU/h)/Watts (ie, 12500/1275 in the questioned case) that will tell you (part of) what you need to know. The rest is up to how much you run it.
It uses 1.275 KWh/h (1275 watts) and that costs you 14.07 cents/hr at 11.04 cents/KWh
Compare to whatever you think you might replace it with - you'd want a higher EER than 9.8, or you could just look at 12500 BTU models and compare watts directly. Less watts costs you less per hour to run - if they are all cooling by the same amount (12500 BTU/hr) then the lowest watts is the most efficient and should have the highest EER as well.
A low-effort search (ie you might find better) shows a current EER of 11.3 on one similar new unit. If exactly equivalent in BTU/h that would be 1106 watts, or 1.106 KWh/h, or 12.21 cents per hour, or a savings of slightly less than 2 cents per operating hour.
If willing to step away from the window-unit format you may be able to get much better EER in a mini-split type system, but it will likely cost more up front.
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Best Answer
If you are not scared of math...
"R-Value" on insulation is SqFt•Hr•Degree F / BTU (USA-units)
So 1 square foot of R1 material can be held one degree different for one hour by one BTU. One square foot of R11 material can be held at 11 degrees difference for one hour by the same one BTU, or 11 square feet of it could be held at 1 degree difference.
My inference is that you are, at present, asking about cooling this storage shed. Taking the very rough approximation that it is a 16x20X16 box, you have 320 square feet of floor, 320 square feet of ceiling, 640 square feet of long side wall and 512 square feet of endwall. This is not an accurate assumption, but it will ballpark fairly well for what I assume is a gambrel roof shed. That is 1792 square feet of surface area. If it's uninsulated, R1 is a good approximation for plywood or similar walls, and you would need 1792 BTUs/hr to maintain 1 degree of difference between inside and outside temperatures. So if you want to cool it to 78F on a 98F day, you'd need roughly 36,000 BTUs/hr.
If you insulated it to R11 on all surfaces, 3,300 BTU/hr would be (approximately) sufficient for the same differential.
There are some adjustments for reality that will alter this a bit:
The standard guide on the side of an air conditioner box conflating BTUs/hr capacity to a certain number of square feet to be cooled is built on a HUGE number of assumptions, and is not very reliable as a result.
The same principles apply to the units the rest of the world uses (watts, meters, degrees C) but the numbers associated change.
Likewise, the same math applies to heating, but the amount you might want to use for a differential temperature may be much larger, depending where you live and how warm you want to keep the structure.