That's a good question. Don't know where you live. How low do you think it's gonna go before the power comes back on?
Other people here will probably have more reasoned and sensible opinions than mine, but what I would do is think about what happens when the power (and therefore the heat) comes back on and the house temperature changes from 48 to 70 in an hour or two. The issue is not how cold it gets, it's how quickly will the guitars be exposed to a change in temperature- What's that gonna do to the finish on your guitars??
Why don't you consider putting the guitars in their cases (even putting them in shipping boxes if you have any around) and treating them like you just got packages on a cold day from UPS; i.e., let the guitars stay in case/boxes for a day and gradually assume the new temperature. Leave them in a closet that doesn't get warm, even!
Of course, you could do it "Chicago Style":
An old swing player who lived in Chicago told me how they used to 'uncork' guitars from their cases on cold days before WWII. You'd get to the club 2 hours early, set your cased-guitar on an empty table and have a drink. After you got through with the drink, you'd go to your guitar and, without opening the case, merely unlatch the case lid. Then, have another drink. After you were through with the second drink, you'd pop the cover on the case, but leave the guitar in the case. Then, have a third drink. By the time you'd finish the third drink, both the guitar and the musician were ready for the Gig!
I think 'Chicago Style' could work for you if you're a slow drinker....