The lock notifies you when the battery power is getting low (supposedly; it’s been a couple of years and their battery hasn’t died yet). So you will have time to change the battery before you get locked out of your cold, dark house.
Though if the power has been off for more than a couple of years, I think you have bigger issues than being able to get inside…
Raises an interesting question for me, though. My keypad opens the garage door. That’s not going to work if there’s no power (rare). If I locked myself out during a power outage, I guess I’d have to go to my neighbor for the extra key.
At the moment, the answer is “other”. My house is currently on the market*, so I could always call my realtor and make him tell me the code for the lockbox, which has a key to the house in it. The formerly-hidden spare key to the house, as a matter of fact.
*In-town move to a more geographically felicitous locale with relation to my husband and I’s places of work. We won’t close escrow for a couple more weeks, but there was no good reason not to put the old house on the market while we prep. Given the average length of time on the market in my current neighborhood, there’s a reasonable shot we’ll have an accepted offer before we close escrow, which would be a load off.