What sensible and latent cooling loads actually measure, why an air conditioner has to handle both, and what happens when equipment is sized without properly accounting for latent load.
Two Different Kinds of Heat
Sensible heat is the heat that changes air temperature — the kind most people intuitively think of as "heat load." It comes from conduction through walls and windows, solar gain through glass, lighting, equipment, and the portion of occupant heat output that raises air temperature. Latent heat is different: it's the energy required to change moisture from vapor to liquid (or the reverse) without changing temperature — in cooling terms, the energy an air conditioner spends condensing water vapor out of humid air onto the cooling coil, which is what actually lowers indoor humidity.
Both are real loads the cooling system has to handle simultaneously, and they don't trade off against each other — a system can satisfy the sensible load (bring the air to the target temperature) while badly underperforming on the latent load (leaving the space humid), producing a room that reads the correct temperature on a thermostat but still feels clammy and uncomfortable.
Where Latent Load Comes From
The dominant sources of latent load in a residential or light-commercial space are occupants (people release both sensible heat and moisture through breathing and perspiration) and infiltration/ventilation air — outdoor air, especially in humid climates, carries significant moisture that has to be removed once it enters the conditioned space. Cooking, showering, and other moisture-generating activities add further latent load, though these are typically handled by localized exhaust ventilation rather than folded into the whole-building load calculation.
Sensible Heat Ratio and Equipment Matching
Sensible heat ratio (SHR) is the fraction of total load that's sensible — SHR = sensible load ÷ total load. A building's calculated SHR describes the split of loads it actually has; an air conditioner has its own rated SHR describing how it splits its cooling capacity between temperature reduction and dehumidification at a given operating condition. Matching equipment SHR reasonably close to the building's calculated SHR matters because a mismatch in either direction causes a real comfort problem: equipment with a much higher SHR than the building needs will satisfy temperature quickly without removing enough moisture (common with oversized equipment, since it short-cycles before it dehumidifies); equipment with too low an SHR relative to the load can over-dehumidify at the expense of running longer than strictly needed for temperature alone.
Why This Matters More in Humid Climates
In dry climates, latent load is often a small fraction of total load, and sizing primarily to the sensible load produces acceptable results. In humid climates (much of the southeastern and gulf coast US, for example), latent load can be a large fraction of the total — sizing equipment purely to sensible load without checking the latent-handling capacity can produce a system that's nominally the "right tonnage" on paper but leaves the space persistently humid, a common source of comfort complaints and, over time, mold/moisture problems in the building envelope.
What This Means Practically
When using a simplified whole-house cooling load calculator, treat the sensible/latent breakdown it produces as informative, not just the total tonnage figure — a load with an unusually low SHR (high latent fraction relative to sensible) is a signal to pay particular attention to the selected equipment's rated SHR at your design conditions, not just its total BTU/hr capacity, and may be a case where consulting a full Manual J and Manual S process (rather than a simplified estimate) is worth the extra step.