I've installed a few battery (only) back-ups. Believe it or not, Briggs and Stratton was the brand I installed. They make battery back up modules for some reason. Anyway, I don't think they are great for a stand alone system. They are super expensive and obviously will run out if there is a long outage (more than a day or 2). But, if your power company charges a higher rate during peak times (3-10 pm), you can program the transfer switch to allow you to use battery power during those times and then charge them with grid power overnight when it's cheaper. So there's that...
If money was no option, the perfect system would be a combo of solar and battery. The panels power your house and charge the batteries during the day and you run off battery power for the evenings/overnight. Wouldn't use much grid power during spring/summer/fall.