Even Manual J (in my quite limited experience) can give wildly inflated results. For example, if a big window has a tree in front of it or a reasonably reflecti

Even Manual J (in my quite limited experience) can give wildly inflated results. For example, if a big window has a tree in front of it or a reasonably reflective or insulating window treatment (and you close it on a hot day!), then your Manual J software may estimate the heat load as if the sun were shining full blast through the glass all the time and give a huge number. I intentionally had a contractor install a unit that was undersized per Manual J in a room, and, as tested during a nearly-record-breaking heat wave, it kept up without coming close to its limits. Fortunately, a lot of modern systems have respectable turndown ratios (better than 5:1 is not that hard to find), so as long as you use controls that can operate the system all the way down to minimum power, oversizing is not that bad.