An older system may be on the full-credit rule, which swaps one kWh sent out for one kWh used later. The rule stays with the house, not the owner, for 25 years. It ends on December 31, 2049 at Mon Power and Potomac Edison, and on March 1, 2051 at Appalachian Power and Wheeling Power.
Ask the utility in writing which rule the house is on. Adding panels that raise output by more than 10% or 1 kW, whichever is greater, moves the house to the new credit. Adding a battery does not. The utility must answer a written question about a planned addition within 10 business days.