In one episode, Penny said that her father's first name was "Bob"...in the episode where her father visits, he tells Leonard to call him "Wyatt". Many people go by their last names with others...so I do believe that, although they haven't officially said so yet, her father's full name is Bob Wyatt...making that her last name too, at least until it becomes Hofstadter. ;)
I have found out Penny's last name!
she married Zack in Las Vegas and his last name is Johnson.
so penny's last name is Johnson because she married Zack.
I found out with help online and the t.v. show
so there you go
Hope this helps!!!!