The amps of output should be written on the unit. They ussually are not more then 20 amps and will regulate themselves down to a trickle. You should be ok with just leaving it on and certainly over night. I do not like the idea of them being plugged in for long periods of time as it can cause corrosion in the boat. I think they call it Galvanic corrosion but I am not sure. Anyhow to be safe I would unplug it in the am and only plug it in when you think it needs it.