You haven't caught on to what I said.
A good aircon is about 250% efficient, whether it is heating or cooling. Yes! 250%! Why, because it is a heat pump. It extracts heat from one source, through a heat exchanger and dissipates that heat through a second exchanger. The medium to transfer the heat from one to the other is a refrigeration fluid (in cars, usually an HFC).which uses the latent heats of vapourisation and condensation under pressure, with the release of the condensed liquid through an expansion valve to generate the coolth. It does not generate heat, it simply transfers the heat from one exchanger to the other. It can therefore be used equally well for heating or cooling.
The compressor in a conventional car is highly inefficient because it has to work with crankshaft speeds of ~800 to ~6000 rpm (through a pulley/belt reduction). If it works while idling, then all the extra energy put into it at full revs is simply wasted. Therefore it wastes a considerable amount of fuel, witnessed by consumption rates rising by typically 1-1.5 l/100 km when you switch the aircon on (perhaps less with larger engines, because they are run at lower crankshaft speeds for longer times).
An electric-driven compressor, which runs at constant speed, is much more efficient, because the motor speed is optimised for that one job. Notwithstanding, the passenger compartment of a car is sheer hell to heat or to cool. In summer, you have vast areas of glass (even IR-reflective) letting in radiation. Round that, you have a thin steel shell covered by a heat-absorbent film of paint and dust/dirt, with microscratches over the whole surface, with almost no insulation, just the trim on the inside. Why does a dog or a baby shut inside a car die within 30 minutes if it is parked in a hot sun with the windows closed? Because the joules entering are ginormous and the aircon has the job of keeping all that heat down to a reasonable level, by pushing those joules to the hot exhanger. In fact, the heat input to the passenger compartment of an average car on a hot summer's day, in the sun, is between 3 and 5 kW.
Conversely, the heat lost when heating a car in the Canadian winter requires more than 5 kW to replace (not many cars have double glazing, do they?).
So let's say, for convenience, we need 5 kW in or out under peak conditions. As I say, a heat pump of the type suitable for this application is 250% efficient, so it requires 2 kW of mechanical energy to turn the compressor, or 2.1 kW of electrical energy. So, under extreme conditions, yes, this is a hefty load on the battery of a hybrid car. But under most normal conditions between Churchill and Phoenix in N. America or between Lulla and the Med in Europe, the duty cycle of the aircon will be much lower than under these extreme conditions (even at 40°C driving in full midday sun, the aircon on my car kicks in for only about 60% of the time to keep the compartment comfortable).
As for home aircon units, I have 1 18,000 BTU (yes, we have not gone metric there!!!
) unit, 2 12,000 BTU and 3 9,000 BTU units in my house. I have studied only the 12,000 BTU units for consumption. The starting surge (at 230 V) is enormous, about 12 A or 2.75 kW, lasting for about 1 second. After that, when it is actually running, it consumes about 1.2 kW but, with a duty cycle of about 65% in hot weather, it averages only about 800 W when on. The real problem with aircon is that they are misused. At the moment, it is just after 11.00 and the temp outside is about 38°C. I am in a small room (~15 m2) with two computers and their peripheries running. This room has a 9,000 BTU unit but it is not switched on. Why? because I'm reasonably comfortable without it. The SE-facing window is shuttered, double-glazed with IR coating. The walls are 16 cm air-channelled bricks. I anticipate I'll need to switch on the aircon for about one hour, this afternoon, when it will probably be ~41-42°C. It is set for 28°C, which is very comfortable for the dry heat we have here. So, to keep this computer room comfortable for my working day, I possibly consume about 500 or 600 Wh of juice. For the whole house, it is doubtful whether we consume more than 1.5 to 1.8 kWh/day under these conditions (exception when my wife is ironing the weekly wash, when she runs the kitchen aircon for about 3 hours). Our neighbour, in a similar house, has 5 split level units on 24/7, set at 20°C, which I consider stupid and energy-profligate - and must make it sheer hell when he goes outside.
I mention this to illustrate that it is easier to remain comfortable in a house than in a car, the latter requiring MORE energy to do so.