Input Voltage: DC 5V / 1000mAh

It should read 1000mA, not mAh. Is the typo yours or theirs?

Anyway, you want to supply the power bank with what it might demand. The 7805 will not be able to handle 1A output if it is fed 18V at the input. It would have to dissipate 13W, which I don't think it can do even with a good heat sink, and it will overheat. (This is just a hunch. With a good heat sink and fan, maybe it will be fine.)

In my experience, it will survive but it will shut itself down when it gets very hot and it will self-limit the current. This works but is a poor design. On the other hand, it might be perfectly adequate for your needs.

Read the data sheet for the 7805. I just looked at the Fairchild version, and it shows example circuits to handle much higher currents.

 

There are many other voltage regulators that will handle 1 A; adjustable, EZ1588CT, 2 A; LM350. The power bank seems to limit input to 5 W so no need to chase efficiency.

 

One of the reasons to use a wide-range DC/DC converter instead of a simple regulator with a source like a solar panel (with a panel voltage not closely matched to the required load voltage) is optimize the power out at the regulated voltage over a much larger range. A 1A current draw at 5vdc output might need 15vdc (less than VMP) at 350mA (less than the IMP) to deliver that power. It won't be MPPT but IV power conversion on the input side will make this circuit charge the fully discharged powerbank much faster in variable light conditions.