The amount of air conditioning (AC) a car needs depends on several factors, including the car’s size, the outside temperature, the number of passengers, and how well the car is insulated. However, there are some general guidelines and key points:

1. Cooling Capacity (BTU/hr): The cooling capacity of an AC system is typically measured in BTU/hr (British Thermal Units per hour). Most car AC systems range between 15,000 to 30,000 BTU/hr, depending on the vehicle’s size.
2. Compressor Size: The compressor, which is the heart of the car’s AC system, is usually rated in tons. A typical passenger car has an AC compressor ranging from about 0.5 to 2 tons. Larger vehicles, such as SUVs or trucks, might require a larger compressor.
3. Car Size: Smaller cars require less AC capacity compared to larger vehicles. For example, a compact car might need around 15,000 BTU/hr, while a large SUV or van might require closer to 25,000-30,000 BTU/hr.
4. Insulation and Window Tinting: Better-insulated cars or those with tinted windows will retain cool air better, reducing the need for a more powerful AC system.
5. Climate: The external temperature plays a huge role. Cars in hotter climates may need a more powerful AC system to maintain comfort.
6. Ventilation and Air Circulation: Good air circulation within the car also helps in reducing the load on the AC system, making it more efficient.

In summary, while there’s no one-size-fits-all answer, understanding these factors can help determine the right amount of AC for a specific car. For most passenger vehicles, a properly functioning AC system that provides between 15,000 to 30,000 BTU/hr should be sufficient.

Here are a few factors to consider:

1. Comfort: This temperature range is considered comfortable for most people, preventing it from being too cold or too warm inside the car.
2. Efficiency: Setting the AC at around 24°C (75°F) is often a good balance between comfort and fuel efficiency. Lower temperatures can put more strain on the AC system and increase fuel consumption.
3. External Temperature: If it’s extremely hot outside, you might need to set the AC slightly lower, but it’s usually unnecessary to go below 20°C (68°F) unless you prefer a very cool interior.
4. Humidity Control: The car AC also helps reduce humidity inside the car, which can make the environment feel cooler even at higher temperatures.
5. Health Considerations: Keeping the AC too cold can cause discomfort, especially when transitioning from the hot outdoors to the cold car interior. It can also lead to potential respiratory issues for some people.

Adjusting within this ideal range should keep the cabin comfortable and the AC system running efficiently.