What size AC for Lafayette, CA? 2.5 tons for 2,000 sq ft

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

2.5 tons central AC or heat pump, about 31,100 BTU/hr of load

The 20 BTU per square foot chart would put in 3.5 tons, about 35% 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 2.5 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 772 square feet in this climate. For a single room, size by the room: 15.6 BTU/hr per square foot.

The climate it is sized for

Climate from NOAA's 1991 to 2020 normals at Concord Buchanan Field (7.9 mi away). Jul is the hottest month, averaging a high of 88°F and a low of 58.7°F, with about 51 days a year at 90°F or more. That gives a design temperature of 94°F against 75°F inside.

What it costs to run

About 1,407 kWh a year at SEER2 14.3, or roughly $458 at California's average residential rate of 32.58¢ per kWh. Cooling runs 6 months of the year here.

What drives the load in Lafayette

Of the 31,100 BTU/hr, 13,680 comes through walls, roof and windows on a 94°F afternoon (44%), 13,000 is sun on the house (42%), 2,120 is four people and their appliances, and 2,300 is moisture the coil has to wring out of Lafayette's summer air (7%).

Heat through the shell is the biggest share here, so insulation and air sealing cut the size you need more than shading would. Sensible heat totals 28,800 BTU/hr before the moisture allowance.

Other house sizes in Lafayette

Single rooms with a window or mini-split unit: a 150 sq ft bedroom needs about 2,300 BTU/hr; a 350 sq ft living room needs about 5,500 BTU/hr; a 600 sq ft open plan kitchen and family room needs about 9,400 BTU/hr.

Month by month in Lafayette

Aug alone uses 25% of the year's cooling energy. The unit runs the equivalent of 647 hours a year at full output.

Lafayette against nearby cities

Of 376 California cities here, Lafayette has the 186th highest cooling load. The range runs from Corte Madera at 15,700 BTU/hr to Palm Desert at 59,000.

Where Lafayette's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is Concord Buchanan Field, 7.9 miles from the center of Lafayette at 23 ft above sea level. That is close enough that the station's summers are Lafayette's summers.

9 other cities use the same station and get the same climate inputs. The closest to Lafayette: Orinda (3 mi away, 9.9 mi from the station), Walnut Creek (4 mi away, 6.2 mi from the station), Pleasant Hill (5 mi away, 2.9 mi from the station), Martinez (7 mi away, 3.4 mi from the station), Concord (9 mi away, 3.1 mi from the station). Lafayette is farther from the station than 5 of them.

The sun over Lafayette in late July

At 37.89°N, the noon sun on 21 July stands 72.5° above the horizon over Lafayette 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.

25,371 people live in Lafayette, the 274th largest of the 376 California cities sized on this site. Coordinates used: 37.8910, -122.1195.

Lafayette AC sizing questions