What is the reward for supererogatory prayers (Nawafil) at home?

Voluntary prayers at home are better than in the mosque, due to the Prophet's (peace be upon him) saying: "Perform prayers in your homes, for the best of a man's prayers is in his home, except for the obligatory ones," and his saying: "Perform some of your prayers in your homes and do not turn them into graves." It has been reported that "the virtue of performing voluntary prayers at home over performing them in the mosque is like the virtue of congregational prayer over praying alone."