We can theoretically make a lot of simultaneous computations but are we sure we will not need exponentially more energy or time to get the result of computation when we add more qubits in future quantum computers ?
The minimum free energy that must be expended to operate a computer is theoretically very close to zero, since it is not strictly necessary to use irreversible primitives[0], such as NAND gates, which is something that we do in traditional computers. For irreversible elements the theoretical requirement is about kT*ln(2) at temperature T. DNA uses about 100 kT per bit copied.
I really recommend that you take a look at Feynman's introduction to quantum computing[1].
[0] http://www.cs.princeton.edu/courses/archive/fall04/cos576/pa...
[1] http://www.cs.princeton.edu/courses/archive/fall05/frs119/pa...