Prius
People have been using Priuses (Prii?) for backup power for years: Connect a 12v inverter to the starter battery, and run your essential stuff. When the starter battery gets low, the electronics will re-charge the starter battery from the traction battery, and when the traction battery starts getting low, it will automatically start the engine and re-charge it's batteries and turn the engine back off once the batteries are re-charged, all without user intervention,.
My ex-father-in-law started doing this years ago, when having a Prius was still verty much something to brag about.
