What size AC does a 1200 sq ft house in Plymouth, MN need?

Notes

• This is a rule of thumb keyed to Plymouth's climate, not a Manual J. The tonnage above comes from 20 BTU per square foot. That figure is not a lookup: the industry's zone table turns out to be a degree-day table in disguise, so it is interpolated continuously against Plymouth's own 956 cooling degree days — two cities in the same zone 800 degree-days apart do not get the same answer here. What Plymouth actually contributes is the two design temperatures — 90°F in summer and -10°F in winter — and those are the numbers a real load calculation begins from. A house here with new windows and a sealed envelope can come in a full ton under this; a 1960s house with an uninsulated attic can come in a ton over.
• Plymouth is on the dry side — summer nights drop 18°F below the afternoon high, and only dry air lets heat radiate away like that. Latent load is small, so almost all of the capacity above goes into temperature rather than moisture. Two consequences: evaporative cooling is worth pricing here in a way it never is on the Gulf coast, and a slightly undersized unit is more forgiving than it would be in a humid climate, because it will still hold comfort even when it runs continuously on the hottest afternoon.
• 11 days a year reach 90°F here, and the design temperature is 90°F. Equipment is sized for the 1% condition, not the hottest hour on record — which means on the hottest afternoons of the year this system is expected to run continuously and still drift a degree or two above setpoint. That is the intended behaviour. Sizing for the record high instead would leave the unit oversized for the other 361 days, short-cycling through all of them.
• Rounding up to 2.5 tons puts this 25% above the calculated load. Nominal sizes jump in half-ton steps and there is nothing between them, so ask whether two-stage or variable-capacity equipment at this size makes sense — that hardware can run at part load and sidestep the oversizing, which single-stage equipment cannot.
• Running cost, roughly. 956 cooling degree days works out to about 430 equivalent full-load hours here — the number of hours the compressor would run at full output if it ran flat out instead of cycling. At SEER 14 that is 738 kWh a season, about $123 at the national average rate. Moving to SEER 18 would cut it to roughly $96 a year — worth less here than the brochures suggest, because the system simply does not run enough hours to earn the difference back quickly.
• About the climate figures. Everything above is computed from ERA5 reanalysis at Plymouth's own coordinates over 2015–2024, not copied from a table: 7,028 heating and 956 cooling degree days, base 65°F. The zone shown alongside is derived from those degree days against the ASHRAE 169 boundaries — your building department assigns zones by county from older climate normals and may list a different one, so go by the county table when you are pulling a permit. City-centre readings also run a degree or two warmer than the airport station most published tables use.