Cooling load is the total amount of heat that must be removed from a space per hour to maintain a desired indoor temperature. It is measured in BTU/hr or tons of refrigeration and represents everything working against the air conditioner: solar heat through windows, heat conducted through walls and roof, body heat from occupants, heat from appliances and lighting, and moisture in the air that needs to be condensed out.
Sensible Load vs. Latent Load
Cooling load splits into two components that behave differently and place different demands on the equipment.
Component |
What It Is |
What Drives It |
Sensible load |
Heat that raises air temperature. Measured by a thermometer. |
Sun exposure, wall/roof conduction, electronics, lighting, occupants |
Latent load |
Moisture in the air that must be condensed. Does not show on a thermometer. |
Occupant breathing, cooking, showers, outdoor humidity entering through infiltration |
A system sized only for sensible load will bring the temperature down but leave the space humid and uncomfortable. In the southeastern US and Gulf Coast climates, latent load can account for 30% or more of total cooling load, which is why dehumidification performance matters as much as raw BTU capacity in those regions.
Peak vs. Partial Load
Equipment is sized to handle the peak cooling load, the single hottest hour of the year for that building at its design outdoor temperature. But the system spends the vast majority of its runtime at partial load, operating on milder days, cooler hours, or in rooms with lower occupancy than the design maximum. A fixed-speed compressor handles partial load by cycling on and off. An inverter compressor handles it by slowing down, which keeps temperature and humidity more stable and uses less energy per hour of runtime.
Why Cooling Load Varies Between Identical Floor Plans
Two houses built from the same blueprint on the same street can have cooling loads that differ by 20% or more. Orientation matters: a living room with west-facing windows absorbs far more afternoon solar heat than one facing north. Tree cover, window treatments, attic insulation condition, and the number of occupants all shift the number. This is why rule-of-thumb sizing based on square footage alone produces oversized or undersized systems more often than it gets the answer right.