What size AC does a 2000 sq ft house in Portland, ME need?

Notes

• This is a rule of thumb keyed to Portland's climate, not a Manual J. The tonnage above comes from 16.6 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 Portland's own 503 cooling degree days — two cities in the same zone 800 degree-days apart do not get the same answer here. What Portland actually contributes is the two design temperatures — 86°F in summer and -1°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.
• Portland is on the dry side — summer nights drop 19°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.
• 4 days a year reach 90°F here, and the design temperature is 86°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.
• Bigger is not safer. An oversized compressor reaches setpoint and stops before it has run long enough to dehumidify, then starts again minutes later. Short cycling is the single most common cause of early compressor failure, and it is bought, not suffered.
• Running cost, roughly. 503 cooling degree days works out to about 226 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 536 kWh a season, about $89 at the national average rate. Moving to SEER 18 would cut it to roughly $69 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 Portland's own coordinates over 2015–2024, not copied from a table: 6,591 heating and 503 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.