Lots of them. A few hundreds, I think. Viability may depend on site. But if there's space nearby for a reasonable array, there doesn't seem to be any reason not to use it no matter how small it gets (any size will result in savings), unless your electricity is dirt-cheap, or the place is so infrequently used that for a significant portion of the time, nobody is charging (which would waste a portion of the local generation and worsen the ROI).