On non-all-electric cars (gasoline or diesel) running the heater uses very little energy since the heat is generated by combustion in the engine and would otherwise be wasted in the radiator.

On non-all-electric cars (gasoline or diesel) running the heater uses very little energy since the heat is generated by combustion in the engine and would otherwise be wasted in the radiator. The only energy dissipation is the fan motor which is a fractional HP motor typically 1/25 to 1/10 horsepower (HP). Air conditioning in these nonelectric cars requires a refrigerant compressor that consumes 3 – 5 HP so theoretically the car uses more fuel when the air conditioner is running. If the alternative to using the air conditioner is driving with the windows open, however, the increased drag at highway speeds would likely offset much of the savings gained by not using the air conditioner. Another factor is that most cars sometimes run the air conditioning compressor by default even when the heat is on in order to reduce humidity. The end result is that there are little savings either way unless you turn the AC off and keep the windows closed. All-electric cars are similar except that the heater is electric (either a heat pump or resistance heater) and actually uses more energy than the air conditioning compressor and the colder the ambient temperature, the more energy it takes to heat the car. This often results in a substantial decrease in the range of the car during wintertime operation, more so than running the air conditioner in the summer. Experienced electric car drivers learn to use air conditioning and heat frugally if they wish to maximize range.

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