you can't leave it on all day, it has a shut-off timer. One of its weaknesses, in my opinion. It goes into shut-off mode after 12 (I think?) minutes, but you want to let it heat-soak for a good ten minutes before using it for best performance. I've turned mine on, gone downstairs to make a coffee, and come back to hit it only to find it in "cool-down" mode and having to toggle the temperature again to wake it up. Not the biggest issue, but something that I wish could be disabled by the user. Also: it heats up (and retains heat) better with AC power. I don't see much of a difference between AC power and the battery at the lower teperature settings but in the upper range it definitely heats up (and recovers heat after a long inhale) much faster plugged into the wall.Is it to let it run the whole day or turn on off every use?