Fuel Master device to turn off alternator and a/c on your car for powre and mileage.
Shows alternator can draw 36.7 % of available powre at idle.
(I didn't see mention of what engine/car it was based on). Down to about 6% at 2500 rpm.
Page is outdated but information might be worthwhile.
I emailed LONG ago and never got a reply so I don't know if they are even in business. It is however reasonbly simple to use a hobbs switch and relays to turn off the alternator and a/c on most any vehicle.
If I remember correctly the old Honda CRX HF actually turned off the alternator from the factory.