How much additional usage time from the HyperJuice battery for MacBook?

Users can use the following calculation method to find out the approximately additional usage time from HyperJuice battery. 

For example, the MacBook Pro 15" (2011 model) which has 78.5Wh rated internal battery inside, and if it lasts for 3 hours based on the user's computing environment.

The formula to calculate the MBP-100 battery will be 100Wh / 78.5Wh x 3 hours - efficiency lost (20%)= 3 additional hours, which makes the total usage time 6 hours (combine of internal and external battery).