Tom the last thing i read on inverter efficiencies was about as clear as fog. The efficiencies are measured against a load that is the most efficient for the inverter in question. As in if you have a 500w inverter that works the best with a 400w resistive load, that is the efficiency stated. Going to a motor load that is 400w such as your saw would have a different efficiency rating.

Blast, being a chemist this is something you probably already know all of this, but here is a couple of links to battery theory and operation. Basically what kind of battery to chose and how to take care of it so you can get the most out of your inverter.

http://www.uuhome.de/william.darden/dcfaq.htm

http://www.wind-sun.com/Batteries/Battery_FAQ.htm