For 400 square feet, ENERGY STAR recommends a 10,000 BTU air conditioner (its chart row for 400 up to 450 sq ft). Go about 10% bigger for a very sunny room, 10% smaller for a heavily shaded one, and add 4,000 BTU for a kitchen.
- Recommended
- 10,000 BTUENERGY STAR chart
- Power
- 680–917 WEfficient to older
- Per hour
- 11¢–15¢At 16.48¢/kWh
Adjusting for your room
| Room | BTU needed |
|---|---|
| Normal room | 10,000 BTU |
| Heavily shaded (−10%) | 9,000 BTU |
| Very sunny (+10%) | 11,000 BTU |
| 4 regular occupants (+1,200) | 11,200 BTU |
| Kitchen (+4,000) | 14,000 BTU |
Power and running cost
A 10,000 BTU unit draws roughly 680 W (efficient ENERGY STAR model) to 917 W (older model meeting the 2014 federal minimum). At the US average 16.48¢ per kWh, that's about 11¢–15¢ an hour while the compressor runs.
Full sizing chart
| Area to cool | Capacity needed |
|---|---|
| 100 to 150 sq ft | 5,000 BTU/hr |
| 150 to 250 sq ft | 6,000 BTU/hr |
| 250 to 300 sq ft | 7,000 BTU/hr |
| 300 to 350 sq ft | 8,000 BTU/hr |
| 350 to 400 sq ft | 9,000 BTU/hr |
| 400 to 450 sq ft | 10,000 BTU/hr |
| 450 to 550 sq ft | 12,000 BTU/hr |
| 550 to 700 sq ft | 14,000 BTU/hr |
| 700 to 1,000 sq ft | 18,000 BTU/hr |
| 1,000 to 1,200 sq ft | 21,000 BTU/hr |
| 1,200 to 1,400 sq ft | 23,000 BTU/hr |
| 1,400 to 1,500 sq ft | 24,000 BTU/hr |
| 1,500 to 2,000 sq ft | 30,000 BTU/hr |
| 2,000 to 2,500 sq ft | 34,000 BTU/hr |
ENERGY STAR chart, assuming 8-foot ceilings. Heavily shaded: −10%. Very sunny: +10%. More than two people: +600 BTU each. Kitchen: +4,000 BTU.
Questions people ask
What size air conditioner do I need for 400 square feet?
For 400 square feet, ENERGY STAR recommends a 10,000 BTU air conditioner (its chart row for 400 up to 450 sq ft). Go about 10% bigger for a very sunny room, 10% smaller for a heavily shaded one, and add 4,000 BTU for a kitchen.
How many watts does a 10,000 BTU air conditioner use?
Roughly 680–917 W, depending on efficiency (BTU ÷ CEER). That's about 11¢–15¢ an hour at the US average price.
Can an air conditioner be too big for a room?
Yes. An oversized unit cycles off before removing much humidity, leaving the room cool but clammy, and uses more energy starting and stopping.