Positive values represent net purchases of energy; negative values represent sales of excess power across the specified period, though hourly values may vary according to resource output and load. (burlingtonelectric.com)
That's either by regenerative braking or by using excess power from the V8 as a generator. (slashgear.com)
This means that most solar homeowners won't get compensated for excess power, making proper solar system sizing important. (earth911.com)