Excellent, excellent, excellent! Did I say excellent? I have one of these pumps between 2 tanks and it pumps out plenty of air when the power goes out. Pump is quiet, batteries are not expensive, look for them on the internet. You can pick up replacement batteries for under $10....look around. This pump does it's own charging and I also keep a spare battery on a separate charger so I'll have plenty of power if and when I need it. A fresh fully charged battery keeps this pump running for a good 4-6 hours.....and that is on full power....you can set it for intermittent power and the battery will last even longer. Nice pump to have on hand, it really works well.