They might need to reduce their flying time slightly, but you don't need hours and hours of reserve power unless you're flying long distances over the ocean (which requires specially certified equipment anyway). The FAA only requires 30 minutes of reserve fuel, or 45 minutes if flying at night. That is almost certainly enough to get you to the nearest usable airfield.

Also, bear in mind that it's much easier to accurately and reliably measure a battery's state of charge than it is to measure how many gallons of fuel are in an airplane's wing.