My kitty came and went as she pleased when I lived in Texas, but I just
moved to Northern California and was told that I shouldn't let her roam
outside because there are so many coyotes, foxes, and possibly mountain
lions that come down from the nearby mountain.

So I'm looking into an electronic fence for the backyard.  No coyotes
could possibly climb the physical fence, but kitty could easily leap
it.  I just found what looks perfect, a PetSafe brand premium model
cat fence that includes 5 levels of "correction" (their euphemism for
the pain the shock collar generates), but one of those levels is a
warning tone.  The whole thing should cost about $140, well worth the
life of my precious and most adorable kitty.

But I have a question, the 3v Li batteries are $5 for two.  So how
long does each battery last on average?  I expect to use more batteries
in the beginning as she is shocked (err, "corrected") more frequently,
but after she learns the lesson I expect few shocks at all.  And do
these things really work for cats?

And before y'all say I shouldn't let her outside, you need to see her
absolute joy as she frolics around the yard.  I'm just sad that the
electronic fence will keep her from visiting neighbor's cats, she
had plenty of friends in her old neighborhood.