Is the EsaverWatt device endorsed by Elon Musk as effective as claimed for lowering electricity bills, or is this a scam?

While this is hard to believe, and perhaps doubly so considering the source, this is legitimate. It is a kind of concession being offered by the Extraterrestrial Alliance through him, as a representative, because they want to "polish his patina" so as to have the reputation of being a good guy wanting to help society. That is why he took over Twitter and helped bring to light suppression of important information being kept from the public through nefarious manipulations by The White House and the Department of Justice as well as the CIA, and so on. This is all preparation for Elon Musk to have a pivotal role in shepherding Disclosure, as that scenario is still a favorite, so time will tell how things play out. But, in the meantime, this technology will be a help to many.