I don't know this for sure, because the power conditioning gear and service panel aren't visible, but I highly doubt that there is a battery system in this installation, so let's assume we're looking at a typical grid-connected (parallel) set up.
That means that all of the power generated goes to the utility, and none of the power is used by the billboard lighting, which after all is only on at night.
What could be wrong with that?
I suppose nothing, really, except that it goes to the heart of the issue of regulatory fairness and misuse of environmental perception. The sentiment goes: "I have solar PV panels on my home/business/facility, so I'm power/grid/environmentally neutral."
These installations are most certainly not grid neutral. There is still a utility service point, with transformer, distribution, transmission, and power plant infrastructure to support it.
The problem in California is that under net-metering tariffs, the bill board owner could "zero-out" their bill and pay for none of the carrying costs of that infrastructure. They simply have to size the solar PV to generate the same amount of power used by the lighting on an annual basis.
That means the neighbors do pay more, by the way, because they now carry the infrastructure costs. And that is the heart of my criticism, not of solar or distributed generation of any type, but of the manner in which it is treated in the regulatory arena.
Now the electric infrastructure is used at a higher capacity factor than it was before. During the day, when formerly it was idle, it now accepts power into the grid. But if the customer isn't paying for the infrastructure, this benefit is really moot to the utility and to the remaining rate payers.
From an environmental standpoint, you can certainly make a case for this installation. The power used at night (or off-peak in utility parlance) is typically sourced from base-load plants, which in CA are large-scale hydroelectric and nuclear. That means outdoor lighting is pretty much carbon free.
The power generated during the day is not only carbon free, but comes into the grid when peaking plants are needed, so it is offsetting power generation with a carbon content.
The biggest benefit of course is to the owner of the billboard, who now wears an environmental cloak covering what is otherwise a blight on the visual environment. And his neighbors help pay for it!