What size air conditioner do I need?

[PhilT]

I’ll cross link my comment rather than repeat it.

Buying a split-system air conditioner: buy in store, or via an installer?

The purpose of installing a split is relevant. If it is to be the primary heating and cooling unit it should be sized accordingly. If it is to ‘backfill’ on a few hot days it just needs to be able to do the job. 10-20 days operation over a year is going to be a different ‘problem’ than if it is going to be used 180 days+. In my case I have an evap system that is adequate most of the summer, and a hydronic heating system that is very nice, and added the split for the increasing number of hot …