Simple-if you have a 120v a/c appliance that takes 1.2 amps, thats 12 amps at 12v. Figuring on 225 amp hours, that would be 18.75 hours to dead or about 9 hours to 50% charge. If you double the batteries, you can at least double your hours. It is interesting that the more batteries you have the slower each battery will discharge. So three battery banks would work out to 27 hrs worth, but in reality, would be closer to 30 or 32 hours. Anybody tell me why this happens? (just look on battery charts). Good Luck, TomC