People installing solar is exactly what you want to encourage, however.

And the people who save any significant amount of tax by doing so would need fields of the damn things to keep up with their own demand.

P.S. As a commercial venture, it wouldn't work as in my scenario I taxed "electricity intended to charge cars". As a private entity, you'd be paying a ton of VAT etc. on the solar panels in the first place.

Plus, powering a car entirely off-grid means far less demand for motorway services, fuel stations, etc. Win-win.

In time, yes, you may have to adjust the rules slightly, of course. But by then the world is an entirely different place.