For those who are looking at a no cost option, this unit seems to be a favorite. I still wonder how different it would be from the above mentioned Milwaukee MX.
http://www.enjoythemusic.com/superioraudio/equipment/0218/Stromtank_S2500_Battery_Power_Review.htm
http://www.enjoythemusic.com/superioraudio/equipment/0218/Stromtank_S2500_Battery_Power_Review.htm