I admit that I know very little about electricity. :-D
I will say that I know a little bit about churches and I am a big fan of Orthodox art and architecture. Orthodox Christians know how to build a church! :-)
Wikipedia says that St Isaacs was completed in 1858. Do you believe it was an earlier date? It is extremely common for churches to be built (or renovated) on the site of a former place of worship. I don't know anything at all about pre-Christian religion in Russia, but the Romans, after converting to Christianity as the State religion, would convert their temples to churches.
For example: the Umayyad Mosque in Damascus was built on the site of a Christian Cathedral, which was built on the site of the Temple of Jupiter. Same as Hagia Sophia in Istanbul. The original church was built over a pagan temple, then converted to a mosque, then to a museum, and then back to a mosque again.
As the Roman Empire and Christianity spread north through Europe, most important places of worship were converted to churches. So it is very likely that many of these magnificent churches are older than given credit for--or at least the foundations.
Do you know what truly surprised me when I was browsing Wikipedia about St Isaac's??? It says that "Grand Duke George Mikhailovich of Russia" was married there on 21 October 2021, and that this was the first royal wedding in Russia since 1917. !!!!
Is the Imperial family back? Are royal weddings taking place again?