What size AC for St. Petersburg, FL? 3.5 tons for 2,000 sq ft

What size AC a 2,000 sq ft house in St. Petersburg needs

3.5 tons central AC or heat pump, about 39,300 BTU/hr of load

Here the 20 BTU per square foot chart lands close: 3.5 tons against a load of 3.3 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 611 square feet in this climate. For a single room, size by the room: 19.7 BTU/hr per square foot.

The climate it is sized for

Climate from NOAA's 1991 to 2020 normals at St. Petersburg Albert Whitted A (1.2 mi away). Aug is the hottest month, averaging a high of 89.8°F and a low of 78.2°F, with about 71 days a year at 90°F or more. That gives a design temperature of 96°F against 75°F inside.

What it costs to run

About 6,146 kWh a year at SEER2 14.3, or roughly $942 at Florida's average residential rate of 15.32¢ per kWh. Cooling runs 12 months of the year here.

What drives the load in St. Petersburg

Of the 39,300 BTU/hr, 15,120 comes through walls, roof and windows on a 96°F afternoon (38%), 13,000 is sun on the house (33%), 2,120 is four people and their appliances, and 9,100 is moisture the coil has to wring out of St. Petersburg's summer air (23%).

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 30,240 BTU/hr before the moisture allowance.

Other house sizes in St. Petersburg

Single rooms with a window or mini-split unit: a 150 sq ft bedroom needs about 3,000 BTU/hr; a 350 sq ft living room needs about 6,900 BTU/hr; a 600 sq ft open plan kitchen and family room needs about 11,800 BTU/hr.

Month by month in St. Petersburg

Aug alone uses 15% of the year's cooling energy. The unit runs the equivalent of 2,236 hours a year at full output.

St. Petersburg against nearby cities

Of 173 Florida cities here, St. Petersburg has the 124th highest cooling load. The range runs from Miami Shores at 36,500 BTU/hr to Plant City at 44,900.

Where St. Petersburg's weather numbers come from

The nearest NOAA station with full 1991 to 2020 normals is St. Petersburg Albert Whitted A, 1.2 miles from the center of St. Petersburg at 4 ft above sea level. That is close enough that the station's summers are St. Petersburg's summers.

1 other city uses the same station and get the same climate inputs. The closest to St. Petersburg: Gulfport (4 mi away, 5.4 mi from the station). St. Petersburg sits closer to the station than any of them, so its numbers are the best measured of the group.

The sun over St. Petersburg in late July

At 27.76°N, the noon sun on 21 July stands 82.6° above the horizon over St. Petersburg and the day runs 13 h 30 min from sunrise to sunset. With the sun that close to overhead, the roof takes most of the heat at midday and the west wall takes the rest after 3 pm. Shorter days give the house a longer night to shed the day's heat.

267,102 people live in St. Petersburg, the 5th largest of the 173 Florida cities sized on this site. Coordinates used: 27.7627, -82.6441.

St. Petersburg AC sizing questions