What size AC for Atwater, CA? 3.5 tons for 2,000 sq ft

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

3.5 tons central AC or heat pump, about 38,900 BTU/hr of load

Here the 20 BTU per square foot chart lands close: 3.5 tons against a load of 3.2 tons. This is one of the climates the chart was written for.

If a contractor quotes you more than 3.5 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 617 square feet in this climate. For a single room, size by the room: 19.4 BTU/hr per square foot.

The climate it is sized for

Climate from NOAA's 1991 to 2020 normals at Merced Municipal Airport (6.4 mi away). Jul is the hottest month, averaging a high of 96.3°F and a low of 61.7°F, with about 100 days a year at 90°F or more. That gives a design temperature of 104°F against 75°F inside.

What it costs to run

About 2,066 kWh a year at SEER2 14.3, or roughly $673 at California's average residential rate of 32.58¢ per kWh. Cooling runs 6 months of the year here.

What drives the load in Atwater

Of the 38,900 BTU/hr, 20,880 comes through walls, roof and windows on a 104°F afternoon (54%), 13,000 is sun on the house (33%), 2,120 is four people and their appliances, and 2,900 is moisture the coil has to wring out of Atwater'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 36,000 BTU/hr before the moisture allowance.

Other house sizes in Atwater

Single rooms with a window or mini-split unit: a 150 sq ft bedroom needs about 2,900 BTU/hr; a 350 sq ft living room needs about 6,800 BTU/hr; a 600 sq ft open plan kitchen and family room needs about 11,600 BTU/hr.

Month by month in Atwater

Jul alone uses 27% of the year's cooling energy. The unit runs the equivalent of 760 hours a year at full output.

Atwater against nearby cities

Of 376 California cities here, Atwater has the 81st highest cooling load. The range runs from Corte Madera at 15,700 BTU/hr to Palm Desert at 59,000.

Where Atwater's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is Merced Municipal Airport, 6.4 miles from the center of Atwater at 152 ft above sea level. That is close enough that the station's summers are Atwater's summers.

2 other cities use the same station and get the same climate inputs. The closest to Atwater: Livingston (7 mi away, 13.4 mi from the station), Merced (8 mi away, 3.1 mi from the station). Atwater is farther from the station than 1 of them.

The sun over Atwater in late July

At 37.35°N, the noon sun on 21 July stands 73.0° above the horizon over Atwater and the day runs 14 h 12 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.

32,248 people live in Atwater, the 232nd largest of the 376 California cities sized on this site. Coordinates used: 37.3540, -120.5972.

Atwater AC sizing questions