This is probably best for long term power interruptions and higher wattage needs. They get their power from gasoline or in some cases, propane.
Generators come in many sizes.
Standard sizes are available from catalogs, discount stores, and some hardware stores. Keep in mind that emergency generators require oil and gasoline, so appropriate storage must be provided for both. Check local code requirements. A generator with electric start requires maintenance to its battery. Whereas, some manual start generators take considerable physical energy to start, and should be evaluated before purchasing.