A. I've had electrical outages from storms several times, not a problem to be dark a while.
B. I flower with more than 12 hours dark to begin with, in nature you flower with less that 12 hours light unless you live on the equator & it takes equatorial strains forever to finish flowering. I was taught a trick to flower equatorial sativas on 10 & 14. It worked and now I pretty much do 10 & 14 on everything. In flower, dark is your friend. Sex hormones rise in the dark and they grow in the dark too, contrary to popular belief. It's not all about photosynthesis when growing plants. I've seen grows finished on 9 & 15. If you want a sexing cut to show female or male, what do you put the lights on to find out fast? Not 12 & 12... more like 8 & 16. They show days sooner on less light. There's no rule that says veg. 18 & 6, flower on 12 & 12. There are a million other ways. And the buds grow just as big, I know that from 1st hand experience. I used 11.5 & 12.5 the first time I flowered indoor plants and have tweaked it down ever since. Think outside the box.