I think Lawrence has already given the correct answer, but let me try to make it simpler by 'breaking down' the meaning of the sentence:
(1) you are comparing the velocity of electricity with the velocity of light.
(2) the velocity of electricity is the same as the velocity of light, is the simplest statement of the implied meaning, which we can state in different ways. Your statement:
Electricity has a velocity (that is) as high as light's (is).
(3) If you want to break the sentence down...
Light's velocity is high.
Electricity's velocity is just as high.
Electricity's velocity is as high as light's (velocity) is (high.)
It is acceptable to drop the repeated 'velocity' and 'high' which are implied, and write it as
Electricity's velocity is as high as light's is
Electricity has a velocity (that is) as high as light's is.
Thus your second sentence is grammatically correct, and follows the same structure as these random examples:
A mobile phone is as expensive as a gold watch is.
Tom is as clever as Walter is.
He is as rich as she is.
...and finally, considering that the final 'is' would be just as implied and superfluous (as 'velocity' and 'high' are), you can therefore drop the final 'is' as well, for consistency, and thus you get your own first sentence:
Electricity has a velocity (that is) as high as light's.