Glassdoor reports the average salary of software engineers in SF as being roughly $100k, which is fairly consistent with the experiences of myself and my friends. It's a hefty income, though bear in mind that the cost of living in SF is well above the US average.
I would say about $78,000/year. However, it depends on the location of where the job is located.
I agree with Kenneth Pu, the cost of living in San Francisco is significantly higher than most other areas in California, however it should not be too difficult to find savings on this in the outskirts of San Francisco or 15 to 20 minutes outside the city itself. Traffic flowing into and out of the city would be difficult, but not impossible.
Here are a couple of articles to compare in regards to salary.
The importance of comparing multiple sources of information is that they pull their data from multiple sources as well. While the data may vary, they all give you a general idea to go off of. You should also keep in mind that these are Senior Software Engineers. These positions take years of being in the businesses to reach and by the time you have entered the job market, this number could significantly increase. The demand for software engineers is not going to diminish anytime soon.