Air conditioners are rated in BTU, and the intuitive assumption that more is better, or at least harmless, is wrong in a way that costs comfort and money.
What a BTU is
A British Thermal Unit is the energy needed to raise one pound of water by one degree Fahrenheit. For air conditioning, the rating you see is BTU per hour, the rate at which the unit removes heat from a room.
An 8,000 BTU air conditioner removes 8,000 BTU of heat per hour. Whether that is the right number depends on how fast heat enters the room, which depends on its size, its exposure, its insulation, and where you live.
The rule of thumb, and its limits
The Department of Energy’s starting point is 20 BTU per square foot of living space. A 300-square-foot room needs roughly 6,000 BTU.
That figure assumes an average room: 8-foot ceiling, moderate sun, reasonable insulation, temperate climate, two occupants. Real rooms deviate, so the estimate gets adjusted:
| Condition | Adjustment |
|---|---|
| Heavily shaded room | −10% |
| Very sunny room | +10% |
| Ceiling above 8 ft | Scale proportionally |
| Poor or no insulation | +15% |
| Consistently hot climate | +10% |
| Kitchen | +4,000 BTU |
| Each occupant beyond two | +600 BTU |
The kitchen adjustment is large because an oven and a cooktop are heat sources measured in thousands of BTU themselves. The ceiling adjustment is proportional and not fixed because you are cooling a volume, not an area: a 10-foot ceiling is 25% more air than an 8-foot one.
The BTU calculator applies these adjustments and shows which ones it used.
Why oversizing makes a room worse
This surprises people. An air conditioner that is too large doesn’t cool better. It cools worse, and here is the mechanism.
An air conditioner does two jobs: it lowers temperature and it removes humidity. Dehumidification happens because warm moist air passes over a cold coil, water condenses on it, and that water drains away. That takes time. The coil has to stay cold, and the air has to keep moving across it, for long enough to pull meaningful moisture out.
An oversized unit hits the thermostat’s target temperature very quickly and shuts off, a pattern called short cycling. The room reaches 72 degrees in eight minutes and the compressor stops before the coil has removed much water.
The result is a room that is cold and clammy. Humid air feels warmer than dry air at the same temperature, so you push the thermostat lower, which makes the cycling worse. Meanwhile:
- Compressors wear out on starts, not on runtime. Frequent cycling shortens the unit’s life substantially.
- Startup is the least efficient part of the cycle, so more starts means more energy for less cooling.
- Temperature swings widen. Long steady runs hold a room within a degree, and short bursts overshoot in both directions.
A correctly sized unit runs longer at a stretch, holds temperature steadily, and dehumidifies properly. On the hottest day of the year it should run almost continuously. That is the design point and doesn’t mean it is too small.
Undersizing is bad too, just more obviously
An undersized unit runs constantly and never reaches the set temperature. It uses more electricity than a right-sized unit doing the same job because it never gets to stop. It also fails to dehumidify well, for the opposite reason: the coil is under continuous load and the unit is simply overmatched.
Undersizing is the more visible failure, which is why so many people overcorrect into the oversizing problem.
Things the square-footage rule misses
Windows: a wall of west-facing single glazing admits an enormous amount of afternoon heat. Multiple large windows on a sunny elevation justify the +10% adjustment on their own, and sometimes more.
Floor position: top-floor rooms take heat through the roof. Ground-floor and basement rooms stay cooler and often need less than the rule suggests.
Open plan: if the room opens into the rest of the house without a door, you are cooling more volume than you measured. Include the connected space or accept that the unit will underperform.
Appliances and electronics: a room with a couple of gaming machines or a rack of equipment has a continuous heat source that the rule of thumb does not know about.
Portable versus window units
A note on ratings, because it causes real confusion. Portable air conditioners are now often listed with two figures: an older ASHRAE rating and a newer SACC (Seasonally Adjusted Cooling Capacity) rating. SACC is the honest one, and it is typically 20-30% lower than the ASHRAE figure for the same machine.
If you size a portable unit against its ASHRAE number, you will under-cool the room by roughly the gap between the two. Compare SACC to SACC, or window-unit BTU to SACC, and not to ASHRAE.
Single-hose portable units also have an inherent handicap. They exhaust indoor air out of the window, which pulls unconditioned outside air in through every gap in the room to replace it. Dual-hose designs avoid this and perform noticeably better for the same rating.
Summary
- Start at 20 BTU per square foot of the space you are actually cooling.
- Adjust for sun, insulation, ceiling height, climate and occupants, and add 4,000 for a kitchen.
- Don’t round up “to be safe”. Oversizing produces a cold clammy room, more wear and higher bills.
- Compare SACC to SACC on portable units.
- Expect it to run for long stretches on hot days. That is correct behaviour.
The BTU calculator does the arithmetic and prints its assumptions, so you can see exactly which adjustments were applied to your number.