What size AC for San Marcos, CA? 2 tons for 2,000 sq ft

What size AC a 2,000 sq ft house in San Marcos needs

2 tons central AC or heat pump, about 21,500 BTU/hr of load

The 20 BTU per square foot chart would put in 3.5 tons, about 95% more than this climate needs. An oversized unit cools the air fast, shuts off before it pulls moisture out, and leaves the house cold and clammy.

If a contractor quotes you more than 2 tons for an average 2,000 sq ft house here, ask to see the Manual J load calculation behind it before you sign. That works out to one ton per 1,115 square feet in this climate. For a single room, size by the room: 10.8 BTU/hr per square foot.

The climate it is sized for

Climate from NOAA's 1991 to 2020 normals at Carlsbad Mcclellan Palomar Airport (6.2 mi away). Aug is the hottest month, averaging a high of 75.8°F and a low of 65.3°F, with about 3 days a year at 90°F or more. That gives a design temperature of 80°F against 75°F inside.

What it costs to run

About 844 kWh a year at SEER2 14.3, or roughly $275 at California's average residential rate of 32.58¢ per kWh. Cooling runs 4 months of the year here.

What drives the load in San Marcos

Of the 21,500 BTU/hr, 3,600 comes through walls, roof and windows on a 80°F afternoon (17%), 13,000 is sun on the house (60%), 2,120 is four people and their appliances, and 2,800 is moisture the coil has to wring out of San Marcos's summer air (13%).

Sun is the biggest share here, more than heat through the walls, so shading west windows and a light roof cut the size you need more than extra insulation would. Sensible heat totals 18,720 BTU/hr before the moisture allowance.

Other house sizes in San Marcos

Single rooms with a window or mini-split unit: a 150 sq ft bedroom needs about 1,600 BTU/hr; a 350 sq ft living room needs about 3,800 BTU/hr; a 600 sq ft open plan kitchen and family room needs about 6,500 BTU/hr.

Month by month in San Marcos

Aug alone uses 28% of the year's cooling energy. The unit runs the equivalent of 561 hours a year at full output.

San Marcos against nearby cities

Of 376 California cities here, San Marcos has the 304th highest cooling load. The range runs from Corte Madera at 15,700 BTU/hr to Palm Desert at 59,000.

Where San Marcos's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is Carlsbad Mcclellan Palomar Airport, 6.2 miles from the center of San Marcos at 313 ft above sea level. That is close enough that the station's summers are San Marcos's summers.

4 other cities use the same station and get the same climate inputs. The closest to San Marcos: Vista (6 mi away, 4.7 mi from the station), Carlsbad (7 mi away, 0.5 mi from the station), Encinitas (8 mi away, 5.5 mi from the station), Solana Beach (11 mi away, 9.6 mi from the station). San Marcos is farther from the station than 3 of them.

The sun over San Marcos in late July

At 33.13°N, the noon sun on 21 July stands 77.3° above the horizon over San Marcos and the day runs 13 h 52 min from sunrise to sunset. A lower noon sun puts more of the day's heat on east and west walls than on the roof, which is why shading west glass pays off here. Shorter days give the house a longer night to shed the day's heat.

96,620 people live in San Marcos, the 79th largest of the 376 California cities sized on this site. Coordinates used: 33.1326, -117.1699.

San Marcos AC sizing questions