I have often thought about whether its was more energy efficient to run my laptop computer off its battery and then recharge, or just keep it plugged into the outlet. Here’s the answer from the Ask Umbra column on Grist.org:
Simply stated, unplug that laptop whenever you’re not using it, or plug it into a power strip that you flip off when not in use. Whether you run the battery all the way down before charging it doesn’t really matter in terms of energy use…. But if you are one of those people who runs the battery all the way down, you might also have a tendency to leave your laptop plugged in longer than necessary to charge the battery—and that turns your laptop into an energy-sucking vampire. I’d just plug it in when you’re using it or when the battery is in need of some juice.