This User Manual page has a great explanation of the difference between the rate types.  

HOMER uses the rate type NetPurchase for positive net energy purchases (Grid imports > Grid Exports) and NetExcess for negative net energy purchases (Grid imports < Grid Exports). If it uses the same rate for both positive and negative net energy purchases, it uses the rate type NetMeter.  

If you have a lower rate for over-generated energy (Grid imports < Grid Exports), “NetExcess” would be the correct rate type to use.