Alan, I think you have a moneymaking idea.
You should be able to sell tons of them to folks worried about losing power and not knowing about it until it's too late.
You could make it 'chirp' like the battery powered smoke alarms so the battery would last longer when sounding the alarm. Once every minute or so would be noticeable but not annoying.
Just remember to include a switch to shut it up in case the whole neighborhood is out to keep from driving everyone in the house nuts.
