No doubt that a fuel-powered system can provide more long-term potential. Car or generator (see my link), doesn't make a difference. But: These batteries kick in anytime they're needed, i.e. at night or when I'm not at home, which is the whole point.
You could go to invertersrus.com and pick up some lifeline batteries and a pure sinewave inverter and have your tank run for 10-20 hours on one charge.
This is what I was going to do. It costs some $$ to do it right though.