The wonders of an adjustable vacuum switch: 
(especially for small engined cars)
When you accelerate you open a throttle valve/s in the engine air intake.
When you brake, your foot is off the gas and that throttle valve is closed.
The engine's rotational momentum pulls a large vacuum between the intake valves and throttle valve.
Enough to power your brake assist booster.
ie:
Whenever you lift off the accelerator and have excess momentum=free power; there's this vacuum. The vacuum will remain/persist if you gear down when slowing, which turns forward momentum into rotational momentum which can/will turn your alternator and aircon pump with free power...
So; you can use this LACK of vacuum during acceleration and an adjustable vacuum switch to auto shut off power to your car's alternator and/or aircon pump while accelerating
and
auto turn them on again when slowing/gearing down and braking.
The 'adjust' lets set when they come on so they don't remain off at cruise.
NB that the above can easily get an auto override so the battery continues charging as usual if its too flat
and the aircon remains on if its too hot.
Adding that too a/the simple circuit diagram is easy with a voltage sensor switch and thermostat switch.
(The 'econ' setting on aicons may already do this to the aircon pump nowadays?? I need to look that up)
The same system can be used to 'auto hot air intake' (economy) at cruise and slowdown and 'auto cool power!' during acceleration.
These have likely been discussed here often. IIRC aerohead does this?
The vacuum takeoff pipe that goes to the brake booster is existent and thinner and most likely easier to T into for said vacuum switch.
Various other switches (interior thermostat, battery level) are most likely also existent.