Last night we experienced a Brownout due to the extended heat and overuse of the power grid here in the Boston area. I first noticed in the afternoon the AC in our office pump was working really weirdly and would "cycle hard" like it was on reduced voltage.
The next sign I got that a brownout was occuring is when I tried to turn on our TV - hooked to a small Conext Battery Backup surge protector - because a brownout causes cycling with low voltage, it freaks out most lower end UPS systems. The battery backup said it had been in surge and overuse of the battery strip - both incorrect - but it's because it couldn't keep a charge.
Off-line UPS units (the vast majority of lower cost units) continuously cycle between power line and internal battery operation. UPS batteries (generally sized to provide only 5 - 10 minutes back-up) will soon be discharged, unable to generate additional Back Up power.
We decided to turn off all non-essential equipment and AC units until bed time, but it didn't make much difference, in the end we had to turn off all of them as they didn't really cool or work well, and you can damage the units if they continue to work under low voltage. We got up about 3 am and turned our daughter's AC unit and things were okay. The other weird culprit were the dehumidifiers in the basement (we sound like a power nightmare), but they acted the same. We turned these off as well.
A great article on Brownouts can be found here...and now I want something to regulate the voltage to key items or the house along with some nice big solar panels or something. Sure - turning off really helps and can save your stuff, but if you aren't home or unaware it's happening very BAD stuff can happen.