The short answer
A 350 sq ft room with average sun and 2 people needs about 8,400 BTU/h of cooling in Houston, where the summer design temperature is 97°F, so a 10,000 BTU window air conditioner. In Seattle (85°F) it needs 7,400 BTU/h, a 8,000 BTU unit. To heat the same room with a space heater in Minneapolis, where it falls to -6°F, you need about 1,507 watts, more than one 1,500 watt plug-in heater can give.
The same room in every state
A 350 sq ft room with average sun, 2 people and no kitchen, sized for the climate of each state's largest city. This table is always for a window air conditioner. Needs are in BTU/h.
| Alabama Huntsville | 3A | 91 | 7,900 | 8,000 |
|---|---|---|---|---|
| Alaska Anchorage | 7 | 72 | 7,200 | 8,000 |
| Arizona Phoenix | 2B | 110 | 8,800 | 10,000 |
| Arkansas Little Rock | 3A | 95 | 8,200 | 10,000 |
| California Los Angeles | 3B | 86 | 7,500 | 8,000 |
| Colorado Denver | 5B | 92 | 8,000 | 8,000 |
| Connecticut Bridgeport | 5A | 86 | 7,500 | 8,000 |
| Delaware Wilmington | 4A | 90 | 7,800 | 8,000 |
| District of Columbia Washington | 4A | 92 | 8,000 | 8,000 |
| Florida Jacksonville | 2A | 94 | 8,200 | 10,000 |
| Georgia Atlanta | 3A | 92 | 8,000 | 8,000 |
| Hawaii Honolulu | 1A | 89 | 7,800 | 8,000 |
| Idaho Boise | 5B | 97 | 8,400 | 10,000 |
| Illinois Chicago | 5A | 89 | 7,800 | 8,000 |
| Indiana Indianapolis | 4A | 89 | 7,800 | 8,000 |
| Iowa Des Moines | 5A | 90 | 7,800 | 8,000 |
| Kansas Wichita | 4A | 97 | 8,400 | 10,000 |
| Kentucky Louisville | 4A | 91 | 7,900 | 8,000 |
| Louisiana New Orleans | 2A | 93 | 8,100 | 10,000 |
| Maine Portland | 6A | 84 | 7,400 | 8,000 |
| Maryland Baltimore | 4A | 92 | 8,000 | 8,000 |
| Massachusetts Boston | 5A | 88 | 7,700 | 8,000 |
| Michigan Detroit | 5A | 88 | 7,700 | 8,000 |
| Minnesota Minneapolis | 6A | 88 | 7,700 | 8,000 |
| Mississippi Jackson | 3A | 94 | 8,200 | 10,000 |
| Missouri Kansas City | 4A | 92 | 8,000 | 8,000 |
| Montana Billings | 6B | 92 | 8,000 | 8,000 |
| Nebraska Omaha | 5A | 92 | 8,000 | 8,000 |
| Nevada Las Vegas | 3B | 108 | 8,800 | 10,000 |
| New Hampshire Manchester | 5A | 90 | 7,800 | 8,000 |
| New Jersey Newark | 4A | 92 | 8,000 | 8,000 |
| New Mexico Albuquerque | 4B | 94 | 8,200 | 10,000 |
| New York New York | 4A | 88 | 7,700 | 8,000 |
| North Carolina Charlotte | 3A | 93 | 8,100 | 10,000 |
| North Dakota Fargo | 6A | 88 | 7,700 | 8,000 |
| Ohio Columbus | 4A | 89 | 7,800 | 8,000 |
| Oklahoma Oklahoma City | 3A | 97 | 8,400 | 10,000 |
| Oregon Portland | 4C | 90 | 7,800 | 8,000 |
| Pennsylvania Philadelphia | 4A | 92 | 8,000 | 8,000 |
| Rhode Island Providence | 5A | 86 | 7,500 | 8,000 |
| South Carolina Charleston | 3A | 92 | 8,000 | 8,000 |
| South Dakota Sioux Falls | 6A | 90 | 7,800 | 8,000 |
| Tennessee Nashville | 3A | 93 | 8,100 | 10,000 |
| Texas Houston | 2A | 97 | 8,400 | 10,000 |
| Utah Salt Lake City | 5B | 98 | 8,500 | 10,000 |
| Vermont Burlington | 6A | 87 | 7,600 | 8,000 |
| Virginia Virginia Beach | 3A | 92 | 8,000 | 8,000 |
| Washington Seattle | 4C | 85 | 7,400 | 8,000 |
| West Virginia Charleston | 4A | 89 | 7,800 | 8,000 |
| Wisconsin Milwaukee | 5A | 87 | 7,600 | 8,000 |
| Wyoming Cheyenne | 5B | 88 | 7,700 | 8,000 |
How it works
The formula
For air conditioners we use the ENERGY STAR room air conditioner chart, with its adjustments, plus one of our own for climate. For a heater we work out how much heat the room loses on a design-temperature day.
Window and portable air conditioners
percent = sun (−10% shaded, +10% very sunny) + climate
climate = 1% per °F that the summer design temp is above 92°F, at most ±10%
need = chart × (1 + percent) + 600 × people over 2 + 4,000 if kitchen
- The ENERGY STAR chart runs from 100 sq ft (5,000 BTU) to 2,500 sq ft (34,000 BTU). It assumes an 8 ft ceiling, a room with average sun and two people.
- The chart has no climate. We add a modest one so a room in Houston (+5%) doesn't get the same answer as one in Seattle (−7%). It is 1% per °F away from 92°F, kept within ±10%. It is our rule, not ENERGY STAR's.
- The result is the smallest unit at or above the need. Window units come in 5,000, 6,000, 8,000, 10,000, 12,000, 14,000, 15,000, 18,000, 22,000, 24,000, 25,000 BTU. Above 25,000 BTU use two units or a mini split.
- Portable units are rated by DOE SACC, which counts the heat a portable unit pulls back in through its hose. SACC is roughly 30% to 50% lower than the old ASHRAE BTU on the box. Size on SACC: common ratings are 5,000, 6,000, 7,000, 8,000, 10,000, 12,000. We show the old label as SACC ÷ 0.6, a rough guess.
- Sources: ENERGY STAR room air conditioners, DOE Energy Saver.
Space heaters
BTU/h = UA × (70°F − winter design temp)
watts = BTU/h ÷ 3.412
heaters = watts ÷ 1,500, rounded up
- The room is a square box with 2 exterior walls, windows of 15% of the floor area, and a ceiling under the roof. The other walls and the floor touch heated rooms and lose nothing.
- Insulation sets the U-factors and leakage. Average is R-13 walls (U 0.075), an R-30 attic (U 0.033), double-pane windows (U 0.5) and 0.45 air changes an hour, half of it for two walls.
- The result is the heat to keep the room at 70°F on the coldest 1% of winter hours, with only the heater running. For a room that also gets some heat from the rest of the house, you need less.
- A plug-in heater tops out at 1,500 watts: 1,500 W ÷ 120 V is 12.5 amps, about all a 15-amp circuit should carry. Two heaters on one circuit trip the breaker, so each goes on its own circuit.
Worked example: 350 sq ft in Houston
Window air conditioner, average sun, 2 people, not a kitchen. Summer design 97°F.
- ENERGY STAR chart for 350 sq ft: 8,000 BTU.
- Climate: (97 − 92) × 1% = +5%; average sun +0%. 8,000 × 1.05 = 8,400.
- 2 people is within the 2 the chart allows for, no kitchen: add 0.
- Need 8,400 BTU/h, so the smallest window unit that covers it is 10,000 BTU. A portable would be 10,000 BTU SACC.
- Same room, but very sunny, 4 people and a kitchen: 8,000 × 1.15 = 9,200, plus 1,200 for people and 4,000 for the kitchen = 14,400 BTU/h, a 15,000 BTU unit.
Worked example: space heater, 350 sq ft in Minneapolis
8 ft ceiling, average insulation, winter design -6°F, so ΔT = 76°F.
- Windows 15% × 350 = 52.5 ft²; net walls 2 × √350 × 8 − 52.5 = 246.8 ft²; air leaks 10.5 CFM.
- UA: walls 0.075 × 246.8 = 18.5; glass 0.5 × 52.5 = 26.3; ceiling 0.033 × 350 = 11.6; air 1.08 × 10.5 = 11.3. Total 67.7 BTU/h per °F.
- 67.7 × 76 = 5,100 BTU/h; ÷ 3.412 = 1,507 watts.
- That is more than one 1,500 W heater, so 2 heaters (6.3 amps each).
Too big or too small
- Air conditioner too big: clammy air. It cools the air fast, shuts off, and leaves the room cold and damp.
- Air conditioner too big: short cycling. Frequent starts waste energy and wear out the compressor, and a bigger unit costs more and draws more power.
- Air conditioner too small: it runs all afternoon and the room never gets down to the setting on the hottest days. If the room is a kitchen or gets full sun, step up a size.
- Portable AC: size it on the SACC number. Buying by the old BTU label gives you a unit about 30% to 50% weaker than you think.
- Space heater too small: it runs nonstop and the room stays cool in a cold snap. A heater too big is usually fine for comfort, but it can trip the breaker; never plug a 1,500 W heater into an extension cord or share its circuit.
Questions people ask
What size window air conditioner do I need for a 350 sq ft room?
The ENERGY STAR chart says 8,000 BTU for 350 sq ft. With 2 people and average sun, we add +5% for the hot summers in Houston (97°F), so the room needs about 8,400 BTU/h and a 10,000 BTU window unit. In Seattle (85°F) the need is 7,400 BTU/h, a 8,000 BTU unit.
How many BTU do I need for a 500 sq ft room?
The ENERGY STAR chart gives 12,000 BTU for 500 sq ft. In Houston we adjust to about 12,600 BTU/h, so a 14,000 BTU window unit or a 12,000 BTU (SACC) portable. Add 600 BTU for each person beyond two and 4,000 BTU if it's a kitchen. A room that big is also a candidate for a mini split.
What size space heater do I need for a 300 sq ft room?
For a 300 sq ft room with two exterior walls and average insulation, we get about 1,326 watts on a design day in Minneapolis (-6°F) and 576 watts in Houston (37°F). The old rule of 10 watts per square foot gives 3,000 watts, which is too much for a well-insulated room and too little for a drafty one in a deep freeze. Plug-in heaters top out at 1,500 watts, so one heater is enough in Minneapolis.
Why is my portable air conditioner so weak for its BTU rating?
Portable air conditioners used to be sold by their ASHRAE BTU rating, which ignored the heat the unit pulls back into the room through its hose and the air it draws in. Since 2017 the DOE rates them by SACC (seasonally adjusted cooling capacity), which is roughly 30% to 50% lower. A unit sold as 17,000 BTU on the old label cools about 10,000 BTU/h in real life. Size on the SACC number, not the label. For this room in Houston we recommend a 10,000 BTU SACC portable, around 17,000 BTU on an old label.
Is it bad to buy an air conditioner that is too big?
Yes. An oversized window or portable unit cools the air quickly, shuts off, and leaves the room cold and clammy because it never ran long enough to take moisture out. It also costs more to buy and run. Smaller is better if the room is cool enough on the hottest days.
How many square feet does a 10,000 BTU air conditioner cool?
By the ENERGY STAR chart, a 10,000 BTU unit cools 401 to 450 sq ft, a 5,000 BTU unit 100 to 150 sq ft and a 18,000 BTU unit 701 to 1,000 sq ft. That assumes an 8 ft ceiling, average sun and two people. Hot climates, sunny rooms, kitchens and crowds all push the need up; the chart tops out at 34,000 BTU for 2,500 sq ft.