This is an expression of how much HHO is produced in one minute for each watt of electricity used, an example:
Lets say our generator produces 1 LPM of HHO, that's 1000 ML and uses 20 amps on a 12 volt setup:
12 (volts) times 20 (amps) = 240 watts of power (per minute)
1000 (ML) divided by 240 (watts) = 4.16 ML of HHO produced each minute per watt of power. Hence, this cells MMW is 4.16.
A higher number indicates higher efficiency.
Russ.