How room AC size is worked out
ENERGY STAR publishes a chart that matches room size to cooling capacity, assuming 8-foot ceilings. Find your room's row, then adjust:
+ 600 for each person over 2
+ 4,000 if it's a kitchen
| Area to be cooled (sq ft) | Capacity needed (BTU per hour) |
|---|---|
| 100 to 150 | 5,000 |
| 150 to 250 | 6,000 |
| 250 to 300 | 7,000 |
| 300 to 350 | 8,000 |
| 350 to 400 | 9,000 |
| 400 to 450 | 10,000 |
| 450 to 550 | 12,000 |
| 550 to 700 | 14,000 |
| 700 to 1,000 | 18,000 |
| 1,000 to 1,200 | 21,000 |
| 1,200 to 1,400 | 23,000 |
| 1,400 to 1,500 | 24,000 |
| 1,500 to 2,000 | 30,000 |
| 2,000 to 2,500 | 34,000 |
Where the chart's rows meet (for example exactly 150 sq ft), this calculator uses the smaller size.
Worked example
A 300 sq ft bedroom is in the 250 to 300 row: 7,000 BTU. It gets a lot of afternoon sun, so add 10%: 7,700. Three people usually sleep there, one more than two, so add 600: 8,300 BTU. An 8,000 BTU unit is a little small; the next common size up is the better pick.
Why not just buy the biggest unit?
An oversized air conditioner cools the room quickly and shuts off before it has removed much moisture, so the room can feel cold and clammy. It also cycles on and off more, which wastes energy. An undersized one runs constantly and may never reach the temperature you want.
Heating
ENERGY STAR doesn't publish a heating chart, and heat needs depend heavily on climate, insulation, windows and drafts. The heater option uses a simple rule of thumb you choose, from 20 to 60 BTU per sq ft, so treat it as a rough starting point. For electric heaters, 1 watt = 3.412 BTU per hour, so a 1,500 W space heater puts out about 5,118 BTU per hour.
When to get a proper load calculation
This chart is for a single room and a room air conditioner. For central air, a heat pump, a furnace or a whole house, the right method is an ACCA Manual J load calculation, which accounts for your climate, insulation, windows, air leakage and orientation. A good HVAC contractor will do one before quoting.