# Volts, Amps, Watts, What? from Henry Gifford

February 22, 2010 | Here are the notes I use for teaching about electricity in an architecture college. They are simplifed explanations based on water flow analogy. Good for anyone wanting to understand electricity for any reason, including parallels to markets.

Volts measure the pressure difference that moves a stream of electrons through a wire.

Amps measure the number of electrons passing through a wire, also called current.

Watts measure the rate of work being done at a particular point in time.

All the above are directly analogous to waterflow. What location would you choose for installing a hydroelectric power plant? The Hudson River has many Liters of water flowing, but with very little pressure because the water falls only a few meters along its course. Forget it– The pipe draining rain from the roof of an apartment house has higher pressure than the Hudson River because the water falls many more meters, but it's no good either because it has only a few Liters of water flowing through it.

The Colorado River has both high flow and high pressure. Therefore it is a reasonable place to install a power plant if you don't care about fish or swimming. This is another way of saying that the Colorado River has a significant amount of energy to start with. If you want to actually measure the energy in the river to compare it to another river you need to know two things: the pressure and the volume of water available. If you multiply the pressure by the volume you get a number that can be used to compare the power in one river to the power in another.

Amps of electricity are like Liters of water: lots in the Hudson, but few in the roof drain.

Volts are like pressure: lots of pressure in the drain pipe but very little in the Hudson.

Watts is a measure of power in either a river or a wire. Power is the ability to get work done, like lifting a weight up a hill, or heating water, or lighting a light.

The high voltage electricity for a car's spark plugs gives you a nasty shock if you touch it because the high pressure (voltage) goes right through your skin, but there's not enough power there to keep a light bulb lit. The D cell in a flashlight puts out only 1.5 volts, which can't push through your skin, but it can light a bulb because it puts out enough power. So how is power measured? Power is measured in Watts. Just as Hydroelectric power is dependant on both pressure and volume, electric power depends on both Volts (pressure) and Amps (volume).

If you measure the number of amps flowing through a wire and measure the number of Volts pushing the electrons through the wire and multiply the two together you get the number of Watts. In other words a wire needs a significant number of both Amps and Volts to have usable power (Watts). The math makes it easy to figure out the Wattage, Amperage, or Voltage if the other two are known.
Amps x Volts = Watts Watts/Volts = Amps Watts/Amps = Volts

Which uses more power? (and puts out more heat)

An electric heater rated at 220 Volts and 10 Amps or a heater rated at 110V at 20A?

220V x 10A = 2,200W 110V x 20A = 2,200W Same in both cases!

What size fuse or circuit breaker can handle the amperage from a 3,000 Watt heater plugged into a 110 Volt outlet?

3,000/110 = 27.3 Amps is the amount of current passing through the wire, so use a 30A breaker.

How much money will it cost to operate a 24Volt transformer to power a thermostat for a year? The transformer is rated at 40Watts (also called 40VA, or Volt-Amps) and electricity in New York City costs about 14 cents for a Kilowatt-Hour:

A Kilowatt-Hour is 1,000 watts used for an hour, or any equivalent such as 100Watts for 10 hours, etc. The transformer runs for a year which is (24 hours x 365 days) 8,760 Hours x 40 Watts = 350,400 Watt-hours. Dividing by 1,000 to get Kilowatt-Hours gives 350 Kilowatt-Hours. Since electricity costs 14 cents for each Kilowatt-Hour: 350 x .14 = \$49.00 worth of electricity for the year.

How much money will you save if you replace a B & G 100 with a Taco 007, when both pumps run 1/2 the hours of the year? The B & G 100 uses 201 Watts and the Taco 007 uses 81 Watts. 8,760/2 = 4,380 hours in 1/2 the year. 201 Watts minus 81 Watts = 120 Watts saved. 120Watts x 4,380Hrs = 525,600 WattHours saved which is about 526 Kilowatt Hours x .14 = \$73.64 worth of electricity saved.

Watch out for decimal place errors when dealing with money - 14 Cents is the same as 0.14 Dollars.

Which heats water faster? A 5,000 Watt electric heater or a 40,000 BTU gas fired water heater?

3.4 BTUs = 1 Watt for one hour, and 3.4 BTUs per Hour = 1 Watt. An electric heater is considered 100% efficient while it's running, while a gas fired water heater loses about 20% of it's energy up the chimney while it's running.

The useful output of the gas fire is:

40,000 BTU (input) x .8 (80% efficiency) = 32,000 BTU (to water) The other 20% went up the chimney.

To convert to Watts:

32,000BTU/3.4 = 9,412 Watts. This is almost double the 5,000Watts of the electric heater. Therefore the gas water heater will heat water almost twice as fast.

Useful Facts: One KiloWattHour = 1,000 Watts for one Hour.

One Therm of gas = 100,000 BTU. One Gallon of #2 oil = 140,000 BTU.

The electricity grid in the US is about 31% efficient, so using one unit of electricity is the equivalent of using about three units of fossil fuel at a power plant.

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