That depends on how many cells you want.
1 LiIon cell has 'about' the same power as 1 NiMH cell.

However, it has 1/2 the weight as you said.
Also, LiION lose less energy to heat when they charge/discharge and they can take a deeper discharge, for more cycles. Also due to this, you can do away with the A/C cooling system in the back to make room for more cells.