[dfox]

Difference between messing with mains wiring and messing with high capacity batteries and/or solar panels is that mains always has some kind of fuse somewhere on the power company side and thus you will not see circuit that is completely unfused and will supply high currents until something blows up spectacularly and dangerously. This holds for significantly smaller things than DIY powerwall, eg. car batteries and even batteries for desktop UPSes.

Edit: for instance accidently shorting net 30 to ground when working on car electrics might well end in the car being completely destroyed by ensuing fire (I have friend who totalled his car in this way, he is licensed electrician). On new cars the fuse between 30 and 30a is usually placed directly on the battery to prevent exactly this issue, but even 30a will supply enough current to wreak havoc.

[SwellJoe]

I can't imagine Tesla would ship that battery bank without fuse protection inside (and I imagine it is a "no user serviceable parts inside" kind of design...it looks like a black box device). Every solar deployment design has fuses on either side of the batteries.

[dfox]

One neat engineering trick in tesla battery packs is that the last resort fusing is part of the battery pack construction itself (the spot-welded connection to the actual cell is intentionaly surprisingly weak to act as a last resort fuse)

On the other hand, the difference is in that for the mains there is always going to be some kind of fusing, that is not part of what the tinkerer owns and probably does not even know where it is located. For solar and huge battery packs you own the whole thing and can mess with anything in the system, even more so when you build the thing from individual cells instead of buying COTS solution with bunch of "no user servicable parts inside" labels.