The big secret in a lot of countries where energy and communications are state monopolies or semi-private corporations is that they are LOADED with debt that the govt used as a private slush fund.
If you are looking for "off balance sheet" liabilities in a government, look at its utilities that are exempt from public records laws. This is where a lot of the bodies are buried.
If people start dropping off the grid, states will just convert the power bill to a straight up tax bill, but without the pretense of a service attached to it. There are already precedents for charging electric vehicle taxes to make up for the shortfall in fuel taxes they create. Govts say, "well, if they are using the roads, they should pay for them." except gas taxes go into the slop bucket of general revenue, and the roads have been paid for through other taxes. The largest state expense is employee salaries, yet all road maintenance is done by contractors, so the "roads" thing doesn't really wash.
When Greece was trying to sort out its default, it outsourced tax collection to its utility companies as part of the bailout.
Efficient power storage will be a game changer, like a redrawing of international boundaries game changer.