Energy and Power

It is easy to get confused by power and energy. Power flows, energy accumulates or depletes. Power is like the light out of a flashlight, energy is what is stored in the battery, and the flow of electricity out of the battery is electrical power.

The best units for considering energy and power are the metric units "Joule" and "Watt". A Joule is one Watt for one second. A Watt is the flow of one Joule per second.

9.8 Joules can lift a 1 kilogram weight 1 meter in the Earth's gravity. One Joule will lift a one pound weight about 9 inches. 4184 Joules is 1 food calorie, or 1000 "chemist's" calories. An "AA" battery stores 1000 Joules. Turning a gallon of room temperature water into steam requires 9.8 Megajoules. Orbiting a kilogram of mass at 100% efficiency (which nobody knows how to do) would require about 32 Megajoules. A gallon of gasoline (along with many gallons of oxygen) produces 130 Megajoules. The complete fission of a kilogram of U235 produces 80 trillion Joules ( 80 million Megajoules ).

Power is not energy, but the flow of energy. A 100 watt light bulb turns 100 Joules per second of electric energy into heat (although some of that power is light, very briefly, before it is absorbed and becomes heat). Electrical power is very versatile - it can become light, or airflow, or computation, or material movement, or chemical change - but it all turns into heat, sooner or later. Power can be stored as energy - chemically, mechanically, magnetically, electrostatically - but storage is expensive, limited, and never perfect;y efficient.

When and where power flows matters as much as how much flows. A flash of lightning is a lot of power (very briefly), but the 100 lightning strokes per second somewhere over the 510 million square kilometers of the globe is not the same as concentrated power in one particular spot, available at the toggle of a switch. The sunlight on your roof may (with great expense) be converted with photovoltaic cells into a trickle of electricity, but unless you or somebody else can use it at the instant it is produced, it must either be stored (with great expense) in batteries, or discarded. If someone else can use it, but they are a long way away, most of the power will be lost moving it over transmission lines to them. If the power lines are already overburdened, it is cheaper to throw away the power you produce than to risk burning out the power lines. If the power is produced by wind in the Pacific Northwest, it tends to be counter-cyclic with demand (power in the middle of the night), so it is either thrown away, or the hydro-power dams must be run in wasteful and fish-killing ways to make up for the variation in wind power.

The ability to produce megawatts, at the wrong time or in the wrong place, is pointless. We pay the power and gas companies to deliver power at the moment we want it, not when they find convenient.


All usable energy comes from stars, and nuclear fusion. The power flow from our sun is 1366 watts per square meter, a mixture of visual and infrared light, with a smattering of energetic particles (solar wind) and ultraviolet. If we could store that light in a container, it would become manufactured energy.


All power and energy eventually becomes heat radiation. The sun produces short wavelength heat radiation (including visible light), and 99.9999% of it is scattered into the interstellar void. The visible light reaching earth is either reflected into space, turned directly into heat and radiated, or converted by plants into heat (and food for animals, which becomes heat). In the long run, sometimes with brief delays of a few million years (coal) or billion years (uranium), all energy becomes heat.


PowerEnergy (last edited 2013-02-17 05:19:01 by KeithLofstrom)