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

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

1.5 tons central AC or heat pump, about 19,500 BTU/hr of load

The 20 BTU per square foot chart would put in 3.5 tons, about 116% 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 1,233 square feet in this climate. For a single room, size by the room: 9.7 BTU/hr per square foot.

The climate it is sized for

Climate from NOAA's 1991 to 2020 normals at Watsonville Municipal Airport (1.5 mi away). Sep is the hottest month, averaging a high of 75.3°F and a low of 51.8°F, with about 8 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 158 kWh a year at SEER2 14.3, or roughly $51 at California's average residential rate of 32.58¢ per kWh. Cooling runs 0 months of the year here.

What drives the load in Watsonville

Of the 19,500 BTU/hr, 3,600 comes through walls, roof and windows on a 80°F afternoon (18%), 13,000 is sun on the house (67%), 2,120 is four people and their appliances, and 700 is moisture the coil has to wring out of Watsonville'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 18,720 BTU/hr before the moisture allowance.

Other house sizes in Watsonville

Single rooms with a window or mini-split unit: a 150 sq ft bedroom needs about 1,500 BTU/hr; a 350 sq ft living room needs about 3,400 BTU/hr; a 600 sq ft open plan kitchen and family room needs about 5,800 BTU/hr.

Month by month in Watsonville

Sep alone uses 32% of the year's cooling energy. The unit runs the equivalent of 116 hours a year at full output.

Watsonville against nearby cities

Of 376 California cities here, Watsonville has the 328th highest cooling load. The range runs from Corte Madera at 15,700 BTU/hr to Palm Desert at 59,000.

Where Watsonville's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is Watsonville Municipal Airport, 1.5 miles from the center of Watsonville at 162 ft above sea level. That is close enough that the station's summers are Watsonville's summers.

4 other cities use the same station and get the same climate inputs. The closest to Watsonville: Gilroy (12 mi away, 12.2 mi from the station), Santa Cruz (15 mi away, 13.8 mi from the station), Scotts Valley (16 mi away, 14.6 mi from the station), Morgan Hill (16 mi away, 15.6 mi from the station). Watsonville sits closer to the station than any of them, so its numbers are the best measured of the group.

The sun over Watsonville in late July

At 36.92°N, the noon sun on 21 July stands 73.5° above the horizon over Watsonville and the day runs 14 h 10 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.

51,320 people live in Watsonville, the 175th largest of the 376 California cities sized on this site. Coordinates used: 36.9236, -121.7724.

Watsonville AC sizing questions