Just curious, wouldn't a place that needs parking meters already be on the grid? Assuming we use parking meters to earn back the costs of building the parking lot or to stimulate the use of other forms of transportation than a car in that area.
On the other hand, having autonomous off-the-grid parking meters might be easier to install and maintain by a third party. Plus it allows you to start growing parking meters everywhere, even in the middle of nowhere. Let them pay!
The ones I've seen have been installed as higher tech replacements for traditional mechanical parking meters. Solar power means the old meter head can just be removed from the pole and the new one attached, with no need to worry about running underground electric service where none was before. I'm sure the savings in installation cost more than makes up for the addition of the solar panel.
On the other hand, having autonomous off-the-grid parking meters might be easier to install and maintain by a third party. Plus it allows you to start growing parking meters everywhere, even in the middle of nowhere. Let them pay!