Sizing by capacity at your design temperature
How big a mini-split do you need in the cold?
The short answer
Size from your heat loss at your 99% design temperature, not from the 47°F label. To cover a 24,000 BTU/h load, a unit shaped like the median certified system needs about 24,000 BTU/h of 47°F rating in Miami (56°F design) but 30,860 BTU/h in Fairbanks (-33°F), because the median unit keeps only 88% of its capacity at 5°F.
The median unit's capacity curve
Required 47°F rating, by climate
Ten cities from warmest to coldest of our 121. Same 24,000 BTU/h design load everywhere, so the only thing changing is the cold.
| City | 99% design temp | Needed 47°F rating (BTU/h) | Certified systems that cover the load |
|---|---|---|---|
| Fairbanks, AK | -33°F | 30,860 | 4,119 of 14,941 |
| Bozeman, MT | -6°F | 28,145 | 4,628 of 14,941 |
| Des Moines, IA | 0°F | 27,605 | 5,075 of 14,941 |
| Milwaukee, WI | 4°F | 27,265 | 5,303 of 14,941 |
| Toledo, OH | 7°F | 26,967 | 5,436 of 14,941 |
| Manchester, NH | 9°F | 26,833 | 5,436 of 14,941 |
| Wichita, KS | 13°F | 26,512 | 5,544 of 14,941 |
| Santa Fe, NM | 16°F | 26,262 | 5,550 of 14,941 |
| Salt Lake City, UT | 20°F | 25,954 | 5,599 of 14,941 |
| Huntsville, AL | 25°F | 25,563 | 5,627 of 14,941 |
| Shreveport, LA | 30°F | 25,190 | 5,688 of 14,941 |
| Jacksonville, FL | 36°F | 24,757 | 5,940 of 14,941 |
| Miami, FL | 56°F | 24,000 | 6,996 of 14,941 |
Method: capacity between 5°F and 47°F is a straight line through the two ENERGY STAR test points; above 47°F it is flat; below 5°F it never rises. The 17°F value is plotted as reported but not used, because ENERGY STAR reports it under a different test condition (in the data it is below the 5°F figure for 52% of systems). The curve's 17°F point is the median 47°F capacity times the median 17°F-to-47°F ratio.
Questions people ask
Why not size a mini-split by square feet?
Heat loss depends on insulation, windows and air leakage, not floor area. Size from a Manual J load calculation at your 99% design temperature, then check the unit's published capacity at that temperature.
Is bigger better in cold climates?
Not automatically. Oversizing mostly adds cost and cycling. The right question is whether capacity at your design temperature meets the load, and that is what the table below computes.
What if my design temperature is below 5°F?
ENERGY STAR certifies capacity down to 5°F only. Below that, check the manufacturer's published low-ambient capacity and plan backup heat; our model never lets capacity rise below 5°F.