Is this an alternative to Stromtank and PS Audio ?

It's very simple.....it is the watt hours of battery/batteries you have available divided by the load.  If you system draws a constant 500 watts......then 5000 watt hours of battery (two 200 amp hour batteries....ie 400 amps x 12.6 volts) will let you run at least 8 straight hours without recharging.  Most do not listen that long.  You can also charge while listening but it does not sound as good as disconnecting the charger from the battery.  If your system only draws 150 watts then you do not need much battery.

The only way to know if an inverter system sounds best is to try one.....the info on my site shows a setup that is mindblowing good.

Yes, and mostly no.

All in ones are good sonically (every brand sounds different.....some are OK, some are good and some are great....just like everything) and some are probably equal to the Stromtanks or even better (however, most of them have noisy fans).....see review by Tom Lyle in the link I provide. However, my friend used the 3000 watt Yeti and that was beat by the Ecoflow pro....which was then stomped to death by the Giandel inverter plus LifePo4 batteries shown. He also uses a Puritan line filter after the inverter for even better results.....all inverters still put out some noise.....the Puritan takes it way, way down. This is revolutionary.....you will not believe the transformation of your system doing what my friend did. You can experiment for almost free....the inverters, batteries and Puritan are all returnable.......do you want to blow you mind?

http://tweakaudio.com/EVS-2/Inverter_Power.html