What size AC for San Anselmo, CA? 1.5 tons for 2,000 sq ft

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

1.5 tons central AC or heat pump, about 15,700 BTU/hr of load

The 20 BTU per square foot chart would put in 3.5 tons, about 167% 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 1.5 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 1,526 square feet in this climate. For a single room, size by the room: 7.9 BTU/hr per square foot.

The climate it is sized for

Climate from NOAA's 1991 to 2020 normals at San Francisco Dwtn (16.6 mi away). Sep is the hottest month, averaging a high of 70.2°F and a low of 55.6°F, with about 3 days a year at 90°F or more. That gives a design temperature of 74°F against 75°F inside.

What it costs to run

About 153 kWh a year at SEER2 14.3, or roughly $50 at California's average residential rate of 32.58¢ per kWh. Cooling runs 0 months of the year here.

What drives the load in San Anselmo

Of the 15,700 BTU/hr, 0 comes through walls, roof and windows on a 74°F afternoon (0%), 13,000 is sun on the house (83%), 2,120 is four people and their appliances, and 600 is moisture the coil has to wring out of San Anselmo's summer air (4%).

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 15,120 BTU/hr before the moisture allowance.

Other house sizes in San Anselmo

Single rooms with a window or mini-split unit: a 150 sq ft bedroom needs about 1,200 BTU/hr; a 350 sq ft living room needs about 2,800 BTU/hr; a 600 sq ft open plan kitchen and family room needs about 4,700 BTU/hr.

Month by month in San Anselmo

Sep alone uses 28% of the year's cooling energy. The unit runs the equivalent of 139 hours a year at full output.

San Anselmo against nearby cities

Of 376 California cities here, San Anselmo has the 374th highest cooling load. The range runs from Corte Madera at 15,700 BTU/hr to Palm Desert at 59,000.

Where San Anselmo's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is San Francisco Dwtn, 16.6 miles from the center of San Anselmo at 150 ft above sea level. That is close enough that the station's summers are San Anselmo's summers.

12 other cities use the same station and get the same climate inputs. The closest to San Anselmo: San Rafael (3 mi away, 15.2 mi from the station), Larkspur (4 mi away, 13 mi from the station), Corte Madera (5 mi away, 12 mi from the station), Mill Valley (5 mi away, 11.4 mi from the station), Richmond (12 mi away, 13.1 mi from the station). San Anselmo is farther from the station than 12 of them.

The sun over San Anselmo in late July

At 37.98°N, the noon sun on 21 July stands 72.4° above the horizon over San Anselmo and the day runs 14 h 15 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.

12,645 people live in San Anselmo, the 347th largest of the 376 California cities sized on this site. Coordinates used: 37.9821, -122.5699.

San Anselmo AC sizing questions