If your system is pulling 500 watts at 120V, that's around 4A of line voltage. If you drop down 20% to 100V, the output will happily still pull its regulated voltage, but now the line components are seeing ~20% more, at 5A. For brown out, you need to overrate your components, and/or shut everything off if the line voltage goes too low.

I used to do electrical compliance testing in a previous life, with brown out testing being one of our safety tests. You would drape a piece of cheese cloth over the power supply and slowly ramp the line voltage down. At the time, the power supplies didn't have good line side voltage monitoring. There was almost always smoke, and sometimes cheese cloth fires. Since this was safety testing, pass/fail was mostly based on if the cheese cloth caught fire, not if the power supply was damaged.