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Watt's In a Name?  Posted: October 6, 2010

Volts, Amps, Watts, and Kilowatt-hours

Reprinted with permission from Sam Avery, Avery and Sun Solar Installations: averyandsun.com

Electricity is useful because we can bend it and shape it in so many different ways. The same stuff can bring you daylight at night, music in the morning, and the news at noon. It can heat your house; or cool it: dry your hair, brush your teeth, charge your cell phone, open cans in the kitchen, and make little pictures on a pixel screen. The same stuff! It has so many uses because we are able to chop up into so many different components.

Electricity is electrons moving in a wire, or a current. (There’s also static electricity – the kind that builds up in the cloths dryer, but we don’t have as much use for it.) Amperage is a measure of current, or how many electrons pass a given point. It is analogous to how much water comes out of a pipe over a period of time. Voltage, on the other hand, is pressure, or how much push there is behind the current. If water just falls out on the ground at the end of the pipe it has low “voltage;” if it streams out hard, like out of a fire hose, it has high “voltage.” Wattage is a measure of power, or volts x amps. (Horsepower is another way to measure wattage.) An electrical system with high voltage and low amperage can have the same total wattage as a system with high amperage and low voltage. Electrical transformers can change voltage into amperage or amperage into voltage to prepare the “shape” of wattage for a particular use. A transformer is like the nozzle at the end of your garden hose; you can get the water to squirt farther by screwing the nozzle in, turning flow into pressure,

Almost everything in your house uses the same pressure: 120 volts, though your stove may have a special 240 volt circuit. What this means is that voltage stays the same and each appliance draws as much amperage as it needs to do what it has to do. So, amperage varies according to use. A hundred-watt light bulb, for instance, uses less than one amp (100w/120v = .83 a). An 800w power drill draws 800w/120v = 6,67 amps. Your electric stove may use exactly the same amount of power (watts), but because it is on a 240 volt circuit, will use only half the amperage (800w/240v = 3.33 a). Stoves and power drills obviously take a lot more power than light bulbs, but despite its low power consumption, you can use a lot more electricity with the light bulb. How is this possible?

This is where Kilowatt-hours come in. Where a watt is a measure of power, a watt-hour is a measure of energy. Energy and power are related, but there’s a big difference. Energy is power times time, or watts x hours. (The “kilo” part just means a thousand: there are 1000 watt-hours in a kilowatt-hour.) If you burn a 100 watt light bulb for 10 hours, or a 1 watt bulb on for 1000 hours, you use up a kilowatt-hour. So, if you leave the light bulb on all night, you’re going to use more energy than turning the power drill on for a minute hear and a minute there, even though the drill uses more power at a given moment. It’s the difference between a gallon of gasoline and a gasoline engine: you use up gas quickly in an SUV, but you can use more gas in a moped if you drive it far enough. On the generating end of a photoelectric system, it’s also the difference between the size of your solar array (its wattage) and the hours of sunshine you get. A 4 kilowatt array will generate 20 kwh of energy in 5 hours of direct sunlight.

The electric meter mounted on the side of your house measures kilowatt-hours, or the total energy you consume: the amount of power you use times the length of time you use it. All the wires, boxes, switches, filaments, appliances, diodes, circuit boards, and semiconductors in your house are just there to slice and dice kilowatt-hours into bite-sized volts and amps that you will experience in the form of a meal from the toaster or Willy Nelson on the radio. Your appliances do this by creating resistance, or converting electrical energy back into heat, light, or mechanical motion. (Resistance is measured by yet another unit, the Ohm, or volts / amps.) Unless you like the feeling of an electric shock running through your body, electricity itself does you no good at all. You have to get those flowing electrons to bump into something: vibrate molecules around them to create heat, push something back like an armature or a speaker diaphragm to create motion, or turn into photons to create light.

But what does all this have to do with solar energy? Everything I have said here applies just as well to coal-fired energy.

That, of course, is the point. Electricity is electricity, no matter how it is generated. You can switch to solar, wind, hydro, nuclear, or a hand crank, never knowing the difference in your house. You can burn that 100 watt bulb for 10 hours using a fist-sized lump of coal or using the sunlight falling on a moderate-sized rooftop solar array for about 15 minutes. Willy will keep singing on the radio either way, at least for a while. The only difference is in how long you would like human civilization to last.

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