In Canada, defense of the home is limited to the amount of force needed to remove a trespasser from one's home. Lethal force is not acceptable. The only time that lethal force is acceptable is to protect yourself and others from imminent harm when no other options are available. Basically if you can escape, then you have to do that. And imminent harm is a lot more stringent than merely having an intruder in your home: they have to point a gun at you (or appear to do so) or something of that nature. Use of lethal force to protect real property is not self defense and is not legal. Basically if an armed intruder breaks into your house and starts to steal your Big Screen TV and you blow him away, you will be charged. Similar to a bank robber who merely hands you a note: there's no imminent danger. You would have to prove that something more than a mere robbery happened.
Am I mistaken in thinking that this is very different in the US?
I think this part may be different in Colorado. About a few months ago or so a drunk college girl mistakingly entered a home that wasn't hers. She proceded into the bedroom and got shot by the homeowner. The homeowner was never charged with any crime and the girl was charged with B&E or trespassing. Woman shot after wandering drunk into home pleads guilty | 9news.com