Are BTU's only calculated using square feet?
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 shed won't be airtight, and as conditioned air leaks out,
unconditioned air will leak in, and need to be conditioned, requiring
additional capacity. Approximately 13.9 cubic feet of air weighs 1
pound, and air has a specific heat of 0.24 (it takes 0.24 BTU to
heat/cool one pound of air one degree F) - there are 5120 cubic feet
of air in my simplified model of your building, and if we assume it's
not very tight and has 1 air change per hour, that means you have to
condition 368 pounds of air per hour, at an additional load of 88.4
BTUs/Hr for every degree difference or about 1800 additional BTUs/Hr
for 20 degrees difference - regardless of insulation value (but
subject to changes depending how well you seal the building, thus
altering the rate of air exchange.)
Insulation installed between studs results in a wall assembly with a
total R-value somewhat less than the R value of the insulation
between the studs, since the studs are not as good at insulating as
the insulation is.


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.