Regarding what alfredo says, it is almost true but you need to know the period. I'm involved in a similar project but aplied to gensets. The frequency is not 50 or 60 exactly so
the biggest problem is to determine the period of the signal. To fined the 0 crossings its a hard task. May be as Alfredo says using lo pass filtering can help. If you know the period of the signal then the rest is math !!!!