Budgeting solar is like chopping firewood for winter. You cut as much as you think you need. Do that again. Then the whole process again.

There are so many factors that can diminish the amount of power available at a particular location that the only reasonable thing to do is overbuild. Imagine a line of teslas at an upstate New York charging station. They cannot get to work without power. But it's raining. And it's winter. And it's 6am. And that treeline has grown a foot higher, pushing dawn back an hour. You cannot have too many panels.

I skipped many other multipliers that negate any error in the needed wattage per stall. Top of the list: latitude. Demand will be constant throughout the year, but solar output can be radically different depending on the season. Then, is the horizon flat? If it isn't, then you aren't getting as many hours of sun. Then whether the panels are fixed or sun-tracking (tracking is better but require more space/maintenance). Tracking rigs are also much less space efficient when the sun is low. They end up shadowing each other for much of the day. Access roads, setback from obstructions (treelines) panel output degrading over time, energy losses in/out of batteries ... there are scores factors to undermine the per-acre efficiency of small solar farms located near to consumers. Imho tesla would do far better by building large solar farms in perfect locations and hooking them into the public grid. Panels at the charging stations can be the window dressing.