If you want something expandable and built for this requirement ou of the box you should look at something like a Jackery, or EcoFlow Delta or the like. Expect to pay about 1200 for everything all included.
However you could build your own system for much less. As @ramble81@lemm.ee said you don't want to use a traditional UPS without a power inverter and that'll rob a lot of efficiency.
You would obviously scale this down but here is a pretty good how to that should give you plenty of terms and concepts to research.
https://theperfectadventure.com/diy-power-station-instructions/