A solar backpack is a backpack with a relatively tiny solar panel. It's not designed to power an apartment. It's barely designed to power a laptop. You wouldn't be plugging one of these into the mains.

Solar systems are well understood, as is the business of connecting them to existing power. Magic boxes exist to handle exactly this problem. Connecting them up isn't hard, but installations usually require professional certification to stop people frying themselves and/or their wiring.

DIY solar makes no sense if you're paying your building for unmetered power.

Generally apartment solar can be a nice optional accessory, but very few apartments have the space for a system capable of powering the entire apartment for a significant part of the year.