The major load on an air conditioning system is heat gain from outside the building, primarily solar radiation passing through windows and heat conducted through walls, roofs, and ceilings. This external thermal load typically accounts for the largest share of the total cooling work your AC must perform, though internal heat sources and humidity also contribute significantly. Understanding where this load comes from helps explain why your system works harder on some days than others and why certain upgrades make a bigger difference than you’d expect.
External Heat Gain: The Biggest Factor
Your air conditioner’s primary job is fighting heat that enters from outside. This happens three ways: solar radiation streaming through windows, heat conducting through walls and the roof, and warm outdoor air infiltrating through gaps and openings. Of these, solar heat gain through glass is often the single largest contributor, especially in buildings with large or south-facing windows. Large glazed surfaces in warm climates cause a considerable increase in cooling load because sunlight carries intense energy concentrated in the visible spectrum, and much of it passes directly through glass into your living space.
The roof is another major pathway. On a summer afternoon, roof surface temperatures can exceed 150°F, and that heat radiates downward into your attic and living space for hours. Buildings store thermal energy through their mass, so the peak cooling load inside your home actually lags behind the peak outdoor temperature. Your AC may work hardest in late afternoon or early evening, not at solar noon, because walls and ceilings are still releasing absorbed heat.
Insulation, window coatings, and external shading devices like overhangs or awnings directly reduce this external load. Research on radiant cooling systems has shown that reducing solar gains with external screens and high-performance glass is one of the most effective strategies for lowering cooling demand.
Sensible vs. Latent Load
The total thermal load on your AC system splits into two categories: sensible load and latent load. Sensible load is straightforward heat, the kind that raises a thermometer reading. Latent load is the energy required to remove moisture from the air. Your air conditioner handles both simultaneously. The evaporator coil cools air below its dew point, causing water vapor to condense on the coil surface. That condensation is your system doing latent cooling, which is why AC units produce water that drains outside.
In hot, dry climates like Arizona, the sensible load dominates and the latent load is relatively small. In hot, humid climates like Florida or Houston, the latent load can represent 30% or more of the total cooling work. Removing moisture requires substantial energy. The standard engineering formula shows that latent load scales with both airflow rate and the difference in humidity between indoor and outdoor air. On a muggy day when outdoor humidity is far above your indoor target, your system burns through significantly more energy just wringing water out of the air, even if the temperature isn’t extreme.
Internal Heat Sources
Everything inside your home that generates heat adds to the cooling load. People, appliances, lighting, and electronics all contribute. A single person at rest produces roughly 75 watts of heat. Cooking appliances, especially ovens and stovetops, can push internal gains in a kitchen to 5 watts per square meter or higher during meal preparation. Lighting adds another 2 to 4 watts per square meter depending on the type and density of fixtures, and LED upgrades cut that contribution substantially compared to older incandescent or halogen bulbs.
In a well-insulated modern home, internal gains can actually become a larger proportion of the total load because the external gains have been reduced so effectively. Office buildings with dense computer equipment and many occupants sometimes see internal heat gains of 15 to 30 watts per square meter, which is why commercial buildings often need cooling even in mild weather.
Where the Energy Actually Goes
The compressor is the component that consumes the most electricity in your AC system, using roughly 3,000 to 3,500 watts per hour in a typical residential unit. The indoor blower fan, by comparison, uses around 500 watts. So roughly 85% to 90% of your AC’s electrical consumption goes toward the compressor doing the thermodynamic work of moving heat from inside to outside. The compressor’s workload is directly proportional to the total cooling load: the more heat and moisture it has to remove, the longer and harder it runs.
This is why variable-speed (inverter) compressors have largely replaced traditional single-speed models. A single-speed compressor cycles fully on and off, running at maximum capacity regardless of actual demand. A variable-speed compressor adjusts its output to match the current load, running at lower capacity when the load is light and ramping up only when needed. This matching of output to load is one of the most effective methods for improving efficiency, and it’s a major reason inverter systems now dominate the market.
How Load Affects System Sizing
HVAC professionals calculate the cooling load of your home to determine what size system you need, measured in tons of cooling (one ton equals 12,000 BTU per hour). They account for your home’s square footage, insulation levels, window area and orientation, local climate, number of occupants, and typical appliance use. Getting this calculation right matters more than most homeowners realize.
An oversized system cools air quickly but cycles off before it removes enough moisture, leaving you with a clammy, uncomfortable home. An undersized system runs constantly without reaching your target temperature on the hottest days. The external thermal load, particularly solar gain and insulation quality, carries the most weight in this calculation because it represents the largest and most variable component.
Reducing the Load on Your System
Because external heat gain is the dominant load, the most impactful upgrades target your building envelope. Adding attic insulation, sealing air leaks, and installing low-emissivity window film or double-pane windows all reduce the amount of heat your AC has to fight. External shading on south and west-facing windows is particularly effective since it blocks solar energy before it enters the glass.
On the efficiency side, current federal standards use the SEER2 rating to measure cooling efficiency. The least efficient systems on the market today have a SEER2 of about 13.4, while ENERGY STAR models start at 15.2 and the best available units reach 23.5. The difference between a 13.4 and a 23.5 SEER2 system means the higher-rated unit uses roughly 43% less electricity to handle the same cooling load. Pairing a high-efficiency system with a well-insulated, well-shaded home compounds those savings because the system runs less often and at lower capacity when it does run.