As CEO of a company that makes battery chargers, the question naturally occurs to me, “Where do my products fit in the larger scheme of things?” Since our chargers are merely adjunct devices for the world of batteries, my real question then is “Where do batteries fit in the whole energy food-chain?”

Energy production and storage schemes fall into 3 basic categories. There are mechanical systems that depend on physical movement of objects, chemical systems that rely on re-configuring things at a molecular level, and nuclear systems that involve splitting or combining atomic nuclei. The chemical systems can be subdivided further into those where molecules are re-configured (i.e. something is burned) and those where only electrons are transferred between atoms (batteries and capacitors).

The question of how to compare these disparate schemes turns out to be a bit tricky. This is because there is no single metric that applies well for all systems. The energy of fossil fuels, for instance, is easily specified in Joules per kilogram, whereas a solar panel would best be measured in Watts per square meter. While Joules and Watts are similar (a Watt being a Joule per second), there is no way to convert between kilograms and square meters.

It may be tempting to compare everything on a Watts per dollar basis, but this adds its own distortion to the mix. Cost efficiency is highly dependent on the state of technology, which thereby masks the inherent energy content of the system itself. Such a scheme would put pedal power far ahead of nuclear fusion, for instance.

So, with the caveat that this is a bit of a fool’s mission, let us proceed. Since most systems can practically be expressed in Joules per kilogram, we will go with that as our metric. By applying some reasonable assumptions, we will rope in the outliers for comparison. Here then, is what we find.

Energy Source |
Energy Density |
Relative To Gasoline |

Nuclear Fusion | 8.94x10^{16} J/kg |
1940000000 |

Nuclear Fission | 8.31x10^{13} J/kg |
1810000 |

Burning Compressed Hydrogen | 1.23x10^{8} J/kg |
2.67 |

Burning Gasoline | 4.60x10^{7} J/kg |
1 |

Burning Coal | 2.40x10^{7} J/kg |
0.522 |

Burning Wood | 1.60x10^{7} J/kg |
0.348 |

Human Power^{1} |
4.19x10^{6} J/kg |
0.0911 |

Lithium Ion Battery | 1.80x10^{6} J/kg |
0.0391 |

Alkaline Battery | 6.70x10^{5} J/kg |
0.0146 |

Compressed Air^{2} |
4.42x10^{5} J/kg |
0.00961 |

Flywheel | 4.00x10^{5} J/kg |
0.00870 |

Nickel Metal Hydride Battery | 2.90x10^{5} J/kg |
0.00630 |

Solar Power^{3} |
2.86x10^{5} J/kg |
0.00622 |

Super Capacitor | 1.80x10^{4} J/kg |
0.00039 |

Wind Power^{4} |
9.23x10^{3} J/kg |
0.00020 |

Electrolytic Capacitor | 1.00x10^{3} J/kg |
0.00002 |

Hydroelectric Power^{5} |
1.00x10^{3} J/kg |
0.00002 |

^{1}Based on getting 200 calories (837 kJ) from eating one cup (.2kg) of rice. This measure (perhaps unfairly) categorizes humans as chemical power rather than mechanical power.

^{2}Based on compressing a cubic meter of air (1.20kg) into a 5 liter bottle. This requires 5.30kJ of energy.

^{3}Based on a panel of pure silicon (2.32g/cc) that is 1 meter x 1 meter x 2.5mm thick. Assume 46% efficiency (present record) and cloudless sky with sun directly overhead (1kW/m2) for one hour.

^{4}Based on the dimensions of a Vestas V90 turbine. Each of 3 blades is 44m long x about 1.75m wide (average). Assume that this total area completely absorbs the kinetic energy of each cubic meter of air (1.20kg) moving at 20mph (8.94m/sec).

^{5}Based on water flowing over a 100 meter dam.

I looked up the non-footnoted numbers on the internet. Various sources differ on the exact values, but the ones I list are approximately correct, and they are close enough to make some generalizations.

Certainly nuclear fuels (shown in red) are vastly more energy-rich than all other approaches. Chemical reactions that involve burning (shown in orange) come next. Filling out the bottom of the list are an intermingling of the electro-chemical systems (green) and the mechanical systems (blue).

So where do batteries fit? They hold the top positions among the non-combustion energy sources. In systems where it is important to produce little heat (or noise), they are clearly the way to go.

There are a number of other interesting points to be made from this data, but I’ll save those for future postings.