Car air conditioners
Cabin cooling splits by where the unit fits: under the dashboard, under the seat or on the roof. Compressor versions carry the heat away through a refrigerant circuit, while appliances with a fan function only blow air. Btu per hour describes the heat removed, the watt on the rating plate usually the electrical power drawn. With the engine off, appliances with their own compressor take their current from the battery, so the draw decides how long you can sit parked.
Car air conditioners
A cabin can be cooled in three ways, and the difference lies mainly in where the unit sits. The mounting position decides before the voltage does, because it sets the dimensions, the path of the air and also what has to be drilled. The under-dashboard kit stays entirely inside the vehicle and spreads the cool air through four outlets. The under-seat version at sixty-seven by forty-five by fifty-two centimetres aims at motorhomes, while the roof unit of sixty-seven by sixty-six and a half by twenty-two point three centimetres goes on the outside.
The figures in the specifications meanwhile are not interchangeable. Btu per hour measures the heat removed, the watt usually the electrical power drawn, so the two cannot be compared straight off. Thirteen thousand seven hundred Btu per hour comes to roughly four kilowatts of heat that the unit gets out of the cabin. After conversion both are watts, except that one is the heat removed and the other the electricity used.
Standing apart are the appliances that one listing calls air conditioner, heater, fan and defroster at the same time. These are units of one hundred and twenty to two hundred watts for the car socket, and in cooling mode they work as a fan. A hundred and fifty watts from the car socket drives no refrigerant circuit here, so this type cools only by blowing air. Anyone who wants the temperature below the surrounding air needs a compressor version from the first three groups.
The under-dashboard kit has its compressor driven by the engine of the vehicle, so with the engine off it does not cool at all. Roof units and under-seat units have their own electric compressor, and in those the current comes from the battery with the engine off and not from the alternator. Watts divided by voltage give amps, so a kilowatt at twenty-four volts means over forty amps. How long you can sit parked like that depends on the capacity of the battery, which the description of the unit does not state.
Voltage across the range follows the vehicle: twelve volts, twenty-four volts and, on one under-seat version, forty-eight as well. Control runs from a digital panel through a remote handset to the unit with an inverter compressor and an ABS body. The choice narrows quickly, then, as soon as the voltage of the vehicle and the space for the unit are clear.