What size AC for James Island, SC? 3 tons for 2,000 sq ft

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

3 tons central AC or heat pump, about 35,600 BTU/hr of load

The 20 BTU per square foot chart would put in 3.5 tons, about 18% 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 3 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 675 square feet in this climate. For a single room, size by the room: 17.8 BTU/hr per square foot.

The climate it is sized for

Climate from NOAA's 1991 to 2020 normals at Dwtn Charleston (2.8 mi away). Jul is the hottest month, averaging a high of 87.5°F and a low of 77.1°F, with about 25 days a year at 90°F or more. That gives a design temperature of 92°F against 75°F inside.

What it costs to run

About 3,845 kWh a year at SEER2 14.3, or roughly $611 at South Carolina's average residential rate of 15.9¢ per kWh. Cooling runs 7 months of the year here.

What drives the load in James Island

Of the 35,600 BTU/hr, 12,240 comes through walls, roof and windows on a 92°F afternoon (34%), 13,000 is sun on the house (37%), 2,120 is four people and their appliances, and 8,200 is moisture the coil has to wring out of James Island's summer air (23%).

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 27,360 BTU/hr before the moisture allowance.

Other house sizes in James Island

Single rooms with a window or mini-split unit: a 150 sq ft bedroom needs about 2,700 BTU/hr; a 350 sq ft living room needs about 6,200 BTU/hr; a 600 sq ft open plan kitchen and family room needs about 10,700 BTU/hr.

Month by month in James Island

Jul alone uses 22% of the year's cooling energy. The unit runs the equivalent of 1,546 hours a year at full output.

James Island against nearby cities

Of 42 South Carolina cities here, James Island has the 40th highest cooling load. The range runs from Gaffney at 32,300 BTU/hr to Columbia at 43,100.

Where James Island's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is Dwtn Charleston, 2.8 miles from the center of James Island at 10 ft above sea level. That is close enough that the station's summers are James Island's summers.

2 other cities use the same station and get the same climate inputs. The closest to James Island: Mount Pleasant (7 mi away, 4.1 mi from the station), Charleston (7 mi away, 4.6 mi from the station). James Island sits closer to the station than any of them, so its numbers are the best measured of the group.

The sun over James Island in late July

At 32.74°N, the noon sun on 21 July stands 77.7° above the horizon over James Island and the day runs 13 h 51 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.

12,451 people live in James Island, the 39th largest of the 42 South Carolina cities sized on this site. Coordinates used: 32.7361, -79.9372.

James Island AC sizing questions