I have stuck with one alternator and added an extra battery, which is charged via the EFIS. If the alternator fails (very unlikely on a Rotax) then I have main aircraft power. When this drops below 6.5v the system auto switches to the backup. Total power drain for the full EFIS plus probes etc is 0.9A. Total backup power should give 2.5 – 3 hours. For a VFR only aircraft I deemed this ok.
Rod1