It's pronounced more or less okay, but I guess to a foreign ear it may be hard to hear the difference between -ść ending and -ści ending. The other one is kind of like śći, the i sound must be pronounced. The -ść ending ends with a consonant.
Elektryczność is the umbrella term, and it's probably not used that often, although it definitely sounds perfectly okay here. "Prąd" is the electric current, and this is what is usually used (while English will rather talk of "power" in most cases). If I won't be able to turn anything - lights, TV, computer at my house, I will say "Nie ma prądu" = "There's no power".
I'm not good with noticing the stress, but I think that's actually good - in Polish it's usually on the second-to-last syllable. In Nominative - elektryczność - the last syllable is -ność, in Genitive - elektryczności - the last syllable is -ci. Which is probably the best way to show how they're different.
"potrafię" and "umiem" are generally the same, it's easier to see it on an example like "I cannot read" (Nie [umiem/potrafię] czytać) - those two mean that I do not have the ability to read, I never learned it.
"Nie mogę czytać" may mean that I am not allowed to read, or that I cannot do it (in some mentioned context) because I don't have time for that... perhaps something else.
"Nie mogę tego przeczytać" (perfective) could mean that I am unable to read this because it's too complicated, or perhaps it's written in an illegible way - but "nie potrafię/nie umiem" would also work here.