Ok, What I meant was you would lose the file you were working on IF you hadn't saved it or it became corrupted because of the power cut.
I have a Desktop and it's PSU has failed, possibly because of a power cut, and that's been out of action for a year and 4 months now, so in that respect I have lost files, I had to start from scratch when I got my laptop.
Hopefully my desktop will be in action again soon, but until then, my files are still "lost".
The battery on my Laptop is just basically a backup supply. If the power cuts out, it runs on battery power until I can safely shut it down, or it shuts itself down when battery percentage gets below 10%. My laptop is hardly ever moved or used portably but when I start using my desktop again, I might have to rethink my strategy.