The idea that insurance won't cover fires from unpermitted, DIY stuff is mostly urban legend. Most homeowners' policies are what are called "all hazards" policies. They cover any kind of damage for any reason, with explicit exclusions like war, earthquake, flood. Outright arson / fraud by the policyholder would also obviously be excluded.

Banks would object to policies that excluded unpermitted work, since it's relatively common and this type of exclusion would leave them with lots of exposure.

So, they may decide to drop you like a hot potato afterward, but a fire from your homebrew powerwall would likely be covered. Read your policy fine print to be sure.