Excuse me? You can get off-the-shelf batteries (although LFP batteries with real BMSes are not actually interchangeable, sadly). But you also need the power electronics, and you should not be doing that as a DIY project unless you really know what you’re doing.
I would look to the off-grid/RV space for disruption. For example, here’s a “portable power station” for $600. It costs about 2x as much as a random 1500VA APC UPS I looked it, except that it can deliver twice the actual power (if your UPS is powering a modern power-factor-corrected PSU, you just need watts and the VA isn’t a big deal, and APC is cheating their specs by including unnecessary reactive power capacity in their headline number) and 20x the energy storage and thus runtime. APC tries hard to obscure very low capacity of their batteries.
No one will buy an off-grid setup that can barely power an espresso machine long enough for a single shot.