Not feasible for a single idiot to accomplish, but you're forgetting we live in an age of State-Level Actors who distribute very sophisticated malware to attack

Not feasible for a single idiot to accomplish, but you're forgetting we live in an age of State-Level Actors who distribute very sophisticated malware to attack enemy nation's infrastructure by borking SCADA controllers in e.g. uranium enrichment plants or petrochemical refineries. One may hypothesize about the feasibility of a router-targeting worm (similar to the Moon router worm, targeting Linksys kit) that has a payload that searches for nearby PowerWalls and uses its owners's resources to play guess the password. Getting in wouldn't be fast, but over a period of months such a worm could well gain control over thousands to tens of thousands of powerwalls. At which point, a DDoS attack on national-level grid infrastructure is within reach.