There's only one way, that's to do some math,
which is easy enough.
HERE'S SOME ROUGH CALCS.
Take 3800 btu as roughly = 1kW hour.
16' x 20' x 8' building, pitched roof, no ceiling- IE insulation in rafters....
Assume this is a timber structure with say glass wool or styrofoam infill, well sealed, with an insulated floor. Let floor be solid, with 2" styrofoam and walls + roof be say 6" glass wool or equivalent with vapor barriers-It's very important the insulation won't get wet.
Losses calculated as Watt losses, (do not confuse with BTU 'R' figuring, I assume you want electric heating?)
let roof = 6.0W per deg. F differential
let walls =10.0W "
let floor = 7.0W "
Note this floor is 2" of styrofoam equivalent + concrete as a solid floor,
OR to be as good as the walls if a basement is sited below, ie 6" of glass wool between the joists.
Calculation of solid floor losses is very complex and the above value is a good general figure based on UK Govt. tables.
Openings say 13% of the floor area, allowing 10% for a window (and a way to get in!)
Air changes, let's use one per hour.
Air-losses will be 20W per deg.F differential
Total losses = 43W per deg F differential
Let the comfort level be 70F
Winter, North, let min temp be 2F* (30 FROST)
= a differential of 68F
*This value, for your area, neeeds to be assessed.
Need 43 x 68F differential = 2.9kW HEAT.
Allow 20% additional safety factor,
say 3.5kW = 13334 btu.
Aircon: This is more difficult, as there are variables to consider, such as area of glass, aspect with regard to sun, materials, color, and intended use and traffic into the building increasing the air changes.
But lets say Summer temperature = 100F*
Let airchanges be 2 per hour= 40W/F diff.
= 30x 63 =1890W (7180 btu) to pump heat out.
This is NOT the compressor load as a heat pump is more than 100% "efficient".
There were recent posts on converting values to tons. I have no experience of air-con, (being british) so others will, I'm sure, advise.
I usually work in Metric, but was schooled in btu, sq ft, pounds etc. We had to give all that up and learn it all from scratch when we joined Europe.
Hope this helps,
Wood work but can't!