Late November here in the capital city of Gaspielia: you know it as Los Angeles. A string of sunny days stretches through the fall, and I've been making electricity on the roof with my little 20-watt solar panel. I built a wood frame to support the solar panel and hold a battery; the whole rig is about 2 feet wide by 1 foot high and deep. With 20 watts of power, you could run a compact fluorescent light bulb to pleasantly illuminate a room. But in stand-alone solar electric systems, usually the capture of energy is separated from its expenditure by a half-day's time. In other words, I put the sunny day into a battery and take it downstairs to light the pages of a book at night. I'm looking forward to using my system in the aftermath of the next huge earthquake, when, along with 10 million others, I'll be living off-the-grid for a while.
Back up on the patio, now the sun has gone, but it's not very dark: ten thousand lights across many miles of the LA Basin dance through the cool desert air, countless megawatts of Gaspielia's luminance pouring upwards into the night. The flows of energy entrained by the life of Gaspielia can be measured in watts, since the watt is a unit that holds a dimension of time. When we say the solar panel produces 20 watts, or the light bulb uses 20 watts, we are specifying a certain amount of energy delivered per second; that is the definition of power. Picture your kitchen sink. You turn on the faucet and water flows at 100 milliliters per second; after a minute, the basin contains 6 liters, and you will have to pay for that quantity of water. In the same way, the energy company will charge you for the quantity of electricity that has been consumed, which is counted in units of the kilowatt-hour. The wattage of the light bulb is like the flow rate of the faucet, and you end up with a basin full of kilowatt-hours instead of water. The analogy breaks down here, because it's hard to think of electricity as a substance, but the concept of flow rates is still helpful in this explanation.
Because it seems that electricity, like water, does flow. Here in LA, the water that arrives in a kettle has flowed through aqueducts from impounded rivers hundreds of miles away. The electricity working to heat that kettle has flowed through forests of steel electroducts from power plants as far away as Utah. One generating station there consists of two coal-burning units, each capable of supplying almost one gigawatt of power. That is a big, big river of electricity, one gigawatt: one thousand megawatts; one million kilowatts; one billion watts. It flows through a 500-mile transmission line across the Mojave Desert before it disgorges on LA, disseminating itself into a million households. It works out to a 1-kilowatt stream supplied to each household; tap into that stream for 1 hour, they charge you for 1 kilowatt-hour. It's exactly like renting a power tool on an hourly basis. You rent the saw, you rent the kilowatt: afterward, they are both gone from your life, but the wood has been cut and will remain cut. The work has got done.
Energy is there, in the glowing coals and the rustling leaves and the climbing moon, mundane and obvious despite its incorporeality. Like a clumsy ghost, energy can be perceived only when it bangs into a solid object, and this provides the key to its measurement. First we say that energy is equivalent to a quantity that we name "work", and then we go out and measure the velocity, for example, or the temperature of an object, both before and after some amount of energy has interacted with it. The amount of change that we measure is the amount of work that happened, which equals the amount of energy that was expended. When the crow launches himself and accelerates into flight, chemical energy in fibers of muscle and nerve is transformed into the force of flapping wings, which propels him through the air. The physical change that we observe in the crow is his increase in velocity, and if we knew his mass as well, we could reckon the work he performed and the energy he spent. The bookkeeping for these transactions of energy and work can be done in units of the joule, which serves also to define our unit of power, the watt. The rivulet of electrical energy flowing through a 20-watt light bulb is converted to light and waste heat at a rate of 20 joules per second. That little space heater is sucking in 1500 watts; leave it on for 8 hours one chilly winter evening, you get gouged for 12 kilowatt-hours, perhaps it would feel better to have got 43.2 million joules for your money. The Gaspielian energy corporations would be equally delighted to bill for the megajoules you consume as for their kilowatt-hours.
It's now mid-December, and the first snows have come to the summits of the San Gabriels, one of the mountain walls that try to restrain the sprawl of our megacity. On the roof, lengthening shadows of my cacti time the approach of the winter solstice. The annual minimum of our local solar resource is here, but still the joules pour down abundantly onto my solar panel and lift electrons to higher perches of voltage. A single joule is a small package: about the quantity of energy liberated when a lemon rolls off the countertop and falls to the floor; or the quantity of work necessary to lift the lemon back up. I have a squarish rock, which I found in the Chocolate Mountains, that is about 10 centimeters on a side. Midsummer high noon, when energy from the sun is being maximally delivered, this rock will get hit with 10 joules per second. During 100 seconds, it will experience the input of 1000 joules, which would be sufficient to warm 250 milliliters of water by about 1 degree C., if the radiant energy were to be converted entirely to heat. Under the sun my chocolate rock warms up quickly; it is an efficient, low-tech, zero-maintenance, 10-watt starlight conversion machine. 10 centimeters square, 10 joules per second, 10 watts of power. Run the numbers, there's a large amount of energy falling from the sky.
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