Gasoline is an impressive energy repository

Supplemental material for the Feature Article "Home photovoltaic systems for physicists" PHYSICS TODAY, July 2008, page 42.

Thomas W. Murphy Jr

Ordinary gasoline stores an extraordinary quantity of energy compared with many alternatives. A lead–acid battery, for instance—currently the cheapest alternative for storing electricity in residential-scale photovoltaic systems—stores only 1/500 the energy that can be obtained from an equal mass of gasoline. A gallon of gasoline can provide about 36 kWh of energy, more than a typical house uses in a day. To produce that amount of energy via photovoltaic panels illuminated with 5 peak-equivalent Sun hours per day would theoretically require 72 100-W panels. Actually, 100 100-W panels is a more reasonable estimate after one takes into account such practical limitations as inefficiencies in transporting and storing charge and power losses associated with the high operating temperatures of the PV panel.

One saving grace is that electrical applications tend to be far more efficient than the thermodynamic processes associated with gasoline consumption. All the same, the comparison given here highlights the difficulty we will face in replacing our precious gasoline with renewable sources.