Internally, it takes three 9V alkalines which will power the unit for 10 hours (per Jerry's tests). It also has an AC jack on the left side for connecting an external 9V Lithium, NIMH RC battery pack, etc. With a Walmart 9V Lithium, I bet it powers the Aerco for hours.
The back of the unit has a switch. Left is for running on the external power source, and the Right is for running on the internal Alkalines.