At Hoover Dam the difference in elevation from lake level to the stream below the generator is approximately equal to 750ft. With 100 percent efficient machines, how many kilowatt-hours can be recovered per kilogram of water passing through the system?

The solutions manual for this book shows the following work:

System; the water path through the dam, from inlet to outlet, steady flow

=-.62Wh/kg=-.00062kWh/kg=-2.8*10^-4kWh/lbm

This is negative because it is work flowing out of the system.

This could be the right answer but they seem to pull that -.62Wh/kg out of nowhere. I'll give 5 stars to whoever can answer this problem with a good explanation for how they came to their answer.