You need a load calculation, not a website size. For a labeled illustration only: 400 square feet × 20 Btu/h per square foot is 8,000 Btu/h, and × 25 Btu/h is 10,000 Btu/h. That 20–25 Btu/h starting point is the same manufacturer-and-retailer rule of thumb already used, with a disclaimer, on this site’s general mini-split sizing page. It can ballpark a typical finished room in a mild cooling problem, and it can badly undersize a west-facing room with poor insulation, or oversize a tight, shaded room. Oversizing can hurt humidity control and cycling; undersizing leaves the room uncomfortable at design temperature.
| Starting point | 400 ft² × that factor |
|---|---|
| 20 Btu/h per ft² | 8,000 Btu/h |
| 25 Btu/h per ft² | 10,000 Btu/h |
If the same head must also heat the room in a cold climate, the heating load at design temperature — not the cooling rule of thumb — may set the size. Have an HVAC contractor size the system, and have an electrician confirm the circuit. The operating-cost calculator answers a different question, and the panel calculator answers the service question.
This is not a permit calculation, a utility approval, or a substitute for a licensed electrician, HVAC contractor, or the authority having jurisdiction. Panel Headroom is an independent educational site. It is not a utility, a certification body, or a contractor. Electrical and generator work that is done incorrectly can cause fire, shock, or carbon monoxide poisoning. Use the adopted electrical code and manufacturer instructions; pull permits where required.