http://www.washingtontimes.com/news/2014/apr/12/shapiro-pistorius-and-south-african-law/
Here is a very interesting article about the burden of proof on Oscar Pistorius by Jeffrey Scott Shapiro, a legal analyst for The Washington Times.
This is not a direct quote from the page but my summary of the article.
Shapiro says that even if Oscar Pistorius is acquitted of murder he could still be found guilty of culpable homicide under South African law. Shapiro mentions that Pistorius is charged with premeditated, deliberate murder and that this could mean a possibly mandatory life sentence (up to 25 years unless there are mitigating circumstances) for Pistorius.
I'd like to explain that in South Africa premeditation is not seen as a seperate type of murder charge (as in first or second degree murder) but as an aggravating factor to the charge of murder. Premeditation falls under "Section 51 of the Criminal Law Amendment Act 105 of 1997" which prescribes mandatory minimum sentences for, among other crimes, premeditated murder.
In his article Shapiro explains putative self defense as follows: Pistorius claims the killing was done mistakenly in self-defense because he erroneously believed an intruder was in his home when he fired four shots through a locked bathroom door.
Shapiro then says that in South Africa self-defense is ground of justification (an "affirmative defense"). This means that the defendant admits to the act of killing but not the crime of murder because he or she lacked the required intent. Shapiro notes that this type of defense could be risky because the defendant must actually admit to the act.
"In U.S. criminal cases, where a defendant pleads not guilty and asserts self-defense, the prosecution would still retain the burden of proof to prove the defendants guilt beyond a reasonable doubt, but the burden of production would shift to the defendant," says Shapiro.
So Pistorius has to prove by "a preponderance of the evidence" that he or she had a reasonable belief his or her life was in danger.
Usually this means the defendant must prove that the victim threatened them, but this is not the case in the Pistorious trial. The issue here is OP's mistaken belief that an intruder invaded his home.
Shapiro then explains that OP must convince Judge Masipa that he made an honest mistake in believing that he was firing at an intruder. This is a subjective test where the judge will decide if he personally believed he was firing at an intruder.
If the judge believes him and he is acquitted of murder, Pistorius must now overcome a higher burden. The objective test of convincing the judge that his mistake was reasonable one that a reasonable man would also make. If Pistorius fails to prove that his mistake was reasonable, he faces a possible culpable homicide, conviction, which is the equivalent of manslaughter under U.S. law.