Like most San Francisco homes, mine has no air conditioning. The fog is our HVAC system, and most years it does the job for free. But every so often a real heat wave rolls through, the whole city wilts, and people like me find ourselves shopping for a window unit we will use four days a year — and drowning in BTUs.
The British thermal unit is an old imperial energy unit: the heat needed to warm one pound of water by one degree Fahrenheit, equal to about 1,055 joules. It is the Fahrenheit-and-pound cousin of the calorie, and outside heating and cooling equipment you will rarely meet it.
The rating is really BTU per hour
Here is the quirk that confused me for years: an air conditioner’s BTU rating is not an energy figure at all. It is shorthand for BTU per hour — a rate of heat removal, which makes it a unit of power.
A 12,000 BTU unit moves 12,000 BTU of heat out of your room every hour, which works out to roughly 3.5 kilowatts of cooling. The label says energy; the meaning is speed.
Why bigger is not automatically better
Undersized units fail the obvious way: they run constantly and never catch up on a hot afternoon. But oversizing backfires too. A too-powerful unit chills the air quickly and shuts off before it has time to dehumidify, leaving the room cold yet clammy, and the rapid start-stop cycling wastes energy and wears out the compressor.
A widely used starting point is roughly 20 BTU per hour per square foot, adjusted for conditions:
- Small bedroom (up to ~15 m² / 160 sq ft): around 5,000–6,000 BTU
- Standard bedroom (~20 m² / 215 sq ft): around 7,000–8,000 BTU
- Large bedroom or office (~30 m²): around 10,000–12,000 BTU
- Open living area (~45 m² and up): 18,000 BTU or more
Add capacity for sunny exposures, high ceilings, poor insulation, kitchens, or rooms that regularly hold a crowd; subtract a little for shaded, well-insulated spaces. Climate matters as much as floor area — a chart written for Phoenix is only a first guess here in fog country.
Cooling power is not electrical power
One more relief for the bill-payer in me: a 12,000 BTU unit does not consume 3.5 kW of electricity. Air conditioners are heat pumps — they move heat rather than generate cold — and a decent one moves roughly three or more units of heat per unit of electricity consumed.
That ratio is what efficiency ratings like EER and SEER capture. Two units with identical BTU ratings can have very different running costs, which matters even to those of us who only run them during the occasional October heat wave.
When comparing air conditioners, read the BTU number as cooling speed for your room size, then check the efficiency rating to see what that cooling will cost you every summer.
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