How efficient are inverters in terms of power consumed vs. usable output? Do they vary much by manufacturer/quality? I'm starting to see some fairly inexpensive inverters on the market. Kragen Auto Parts was advertising a Coleman Powermate 800 watt model for $79.99 in our paper this weekend. Since I spend 90% of my time in fresh water I'm not too concerned about it not being made specifically for a marine environment.