In one episode, Penny sinabi 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 sa pamamagitan ng their last names with others...so I do believe that, although they haven't officially sinabi 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!!!!
They will never give Penny a last name because her character is meant to be an archetype, a typical example of a simple, ordinary, mid-western, uncultured girl. Even her backstory is archetypal of the typical, small-town girl who travels to California and has dreams of becoming a movie star. Additionally, her character serves as a contrast, or foil, to the other main characters in the series who all come from important families (hence have family names), live complicated, unorthodox lives and have complex emotional issues.