If it's not something you've actually monitored I don't know what to say.

Does this happen often?

If your power goes out temporarily and you run an oil heater (which stays warm for several hours after it shuts off) I'm not sure you need to be worried about anything. If your power goes out every other day for long periods of time, then I'd consider a generator. Information is everything here, if you want answers to a specific solution, you need to be specific about your problem.