John Sweeney, a Harvard, Massachusetts electrical engineer used his Toyota Prius to power part of his house during a three-day blackout last month.

Mr Sweeney used an inverter to convert direct current from the car battery to 120-volt alternating current for the refrigerator, freezer, the fan for a wood stove, television and some lights. He ran the car for a few minutes every half an hour and “burned about five gallons (19 litres) of gas”.

While knowing that you can use a car as an emergency generator might enhance its appeal enormously, especially when the reliability of the electricity grid is questionable, the main benefit of Mr Sweeney’s use of his Prius was to show the practicality of using the batteries in electric vehicles as a way of storing electrical energy and releasing it as required.

As wind and solar power become more common, having a means of storing energy when electricity is not being generated will becoming critical. One solution which has been proposed is for electricity utilities to contract with motorists to be able to use a small fraction of the power stored in the batteries of parked vehicles.