I have security cameras at my investment property. It is a great thing to have to ensure security and to keep an eye from afar. If you want a live feed, you need it to be hooked up to some wireless provider.
I also included a clause on the lease agreement saying that security cameras are in use, however, they do not ensure safety and will only be used outside.
Realistically, you can tap into their internet. Maybe let them know or give them a few bucks a month so they don't care.
I think they're unnecessary but it depends on your area.
Definitely talk to the tenants and get their permission before installing anything. If it uses their electricity or internet then you should definitely reimburse them. You may be able to give the tenants access to the security feed in exchange for using their electricity and internet.
@Ernest Ho I am in a similar situation debating about installing a security camera for a multi unit on front and back entrance and I am curious about your comment.