What's the point of adding an alternator? If you take out the A/C compressor and replace it with an idler pulley your existing alternator will be providing 20 amps in excess of needs. You'd be better off adding more storage, i.e. put in another battery first. Having two batteries will provide the power even after the engine is shut off and not increase the load on the engine while running giving you better mileage and more power.