Right, I guess I was getting at the 'cost' of energy would be he opportunity cost of not selling it back to the grid which is typically the retail price in area

Right, I guess I was getting at the 'cost' of energy would be he opportunity cost of not selling it back to the grid which is typically the retail price in areas with net metering. Although in off-the-grid situations, you're right that there wouldn't be any incremental cost to charging the storage.