Wouldn't an electric one need a better than stock alternator to recharge the battery again then? The extra strain might just mean the thing will break sooner or later.
I also believe and I speak under correction here, but I believe the more strain you put on the alternator, the more power it zaps from the engine to charge the battery. Reason being that no matter what you do, the energy has to come from somewhere. So the more power the alternator has to provide, the harder it has to turn.
The aircon has a compressor, so an electric one would probably mean that the compressor has it's own electrical motor that will be turned by the battery's power.
I don't know if the power zapped will be less from an electric motor. But I believe the better the aircon works, the more power it will be zap, electrical or not.
The alternator is also not 100% efficient, so basically what you're doing by using an electrical motor is converting kinetic energy from the engine to electrical energy and then using that electrical energy to turn a motor, in other words converting it back to kinetic. Energy is lost in each conversion process, not destroyed(as energy can't be destroyed), but lost through inefficiencies as heat is created which is a waste.
The electrical systems might just be really crap and work badly, so they use less energy and thus don't drain as much power. But why spend money on something that hardly works. If you have to get an aircon, a conventional system will probably work better and be more efficient.
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