How do electric meters work?

July 3, 2006 1:22 PM

How do electric meters work? How do they measure the amount of charge you used? I suspect they must know the amount of charge that enters the home, and then they have some way to measure the amount of charge that leaves the house, but I'm really not sure.

More broadly, since the more I think about it, the more I realize I don't really understand it. How is electricity even converted into mechanical energy like in a motor? Is it all done with magnets or is there some other method at work there?

posted by willnot to technology (18 comments total)

Wikipedia.
posted by phearlez at 1:26 PM on July 3, 2006

Also
posted by b1tr0t at 1:27 PM on July 3, 2006

But how can you read what's coming out of a particular socket?
posted by bingo at 1:36 PM on July 3, 2006

After reading the Wikepedia article, it seems I'm totally backwards on the way electricity works. I thought there was always a fixed current going through the wires in the house, and you draw off some portion of that current.

It looks like plugging something in actually causes more electricity to be (pulled | pushed | ...?) into the home. How does it know I want/need more current when I plug something in?

Also, some electricity is obviously required to move the metallic disk in the induction meter. Is that just a baseline that gets zeroed out of the calculation, or am I charged for that as well.

I clearly don't really understand anything at all about electricity. Ben Franklin, where are you when I need you?
posted by willnot at 1:42 PM on July 3, 2006

Oh, the answers to one of my questions is way down at the bottom of the Wikepedia article:

(it is fraud for it to charge customers for the power meters' consumption). Normally the power supply uses a large capacitor, charged by the high voltage through a diode
posted by willnot at 1:48 PM on July 3, 2006

It looks like plugging something in actually causes more electricity to be (pulled | pushed | ...?) into the home. How does it know I want/need more current when I plug something in?

that's right. people say "drawing current" rather than push/pull but you've got the basic idea. what's fixed is the voltage, and the amount of current that flows is determined by the path from high voltage to low. this is why short-circuiting things is bad - it draws a LOT of current, gets hot, etc.
posted by sergeant sandwich at 1:52 PM on July 3, 2006

I know this is not a perfect analogy, but it helped me. Think of the energy in the electricity lines like water in pipes, running into and through your house. There is always a (relatively) high amount of pressure in the pipes, but no water is being used. Now, when you want some water (for whatever reason) all you have to do is turn the water faucet on and out it comes. You can turn it on a little, and a little water will come out, or you could turn it on full, and have a lot of water come out. There is a fixed voltage (think of water pressure) in your house, but what varies is the current (think water speed and amount). So whenever some appliance needs more electricity, just think of it like it's opening a valve more, so more water (electricity) can flow.
posted by tdreyer1 at 1:54 PM on July 3, 2006

I think you can compare the whole process pretty well to the way your water supply works. Your power company keeps the incoming pipes at a constant pressure (voltage), and normally the outlets are arranged so that no water (current) can flow. When you open a tap (connect a device), the pressure causes a certain amount of water flow depending on the diameter and friction (resistance) of your outlet.
posted by themel at 1:57 PM on July 3, 2006

You may also enjoy Neal Stephenson's Mother Earth, Motherboard essay (warning LARGE page). Although the topic is global telecommunications, there is a long section that deals with the engineering challenges that the designers of the first underwater communications cables faced. Issues like voltage drop-off and induction are covered in the relatively easy narrative.
posted by b1tr0t at 2:01 PM on July 3, 2006

But how can you read what's coming out of a particular socket?

Is this a highly theoretical question or a practical one?

You can measure amp draw with a number of devices so you could put a ammeter on a socket (or the device attached to that socket) and watch the use. You're billed by amp-hours so a little basic math (and some padding to allow for entropy) and you'll know what that costs you.

On a practical level there's devices that will do this, ranging from individual dinguses [review] to things you can wire into the breakers. Some time back I saw an article from someone who used one that installed on individual breakers and reported back to a computer, presumably via X-10, but I can't seem to find the article.
posted by phearlez at 2:15 PM on July 3, 2006

The water analogy is probably good enough. In addition the amount of current drawn by an appliance is determined by its effective resistance. Some let through a trickle, other devices let through a lot.

bingo: But how can you read what's coming out of a particular socket?

Put an inline meter between the socket and the appliances.

willnot: More broadly, since the more I think about it, the more I realize I don't really understand it. How is electricity even converted into mechanical energy like in a motor? Is it all done with magnets or is there some other method at work there?

Electric current acts like a magnet. In straight wires the effect is is fairly small. However, if you coil the wires and wrap them around certain kinds of material, you can increase the effect. This is commonly called an "electromagnet."

An electric motor has a series of electromagnets set up to rhythmically push and pull a rotating magnet. It's like how you get a playground merry-go-round going. You reach out at the right moment, give the handle a pull, then a little push, then get ready to grab the next handle.
posted by KirkJobSluder at 2:16 PM on July 3, 2006

The water analogy is probably good enough.

Right, and think of the meter as something that measures water flow like a little water wheel or something. It doesn't matter exactly what it is, just as long as it measures water flow well. To complete the analogy, Voltage is like pressure, and resistance is like the size of the opening in the faucet, and current is the amount of water that actually flows out.

So if you have a huge opening, but not much pressure, you won't get much flow, If you have a huge pressure and a small hole a lot of water will come spurting out of that hole. The formula used is V = IR. Where I = Current. And R = resistance. When the faucet is turned off, or you don't have anything electrical turned on, then R = Infinity, and so I must equal zero. V is held constant by the power company at about 120 or 240 volts, or other depending on where you live.

Electrical meters measure both voltage and current so that if the voltage drops for some reason, you wouldn't be charged extra, since you are actually paying for energy not current (unlike the water company where you pay for water). Remember E = V*I, so if the voltage doubles and the current halves, you still use the same amount of energy.

Remember all three can change, but V is normally held constant.
posted by Paris Hilton at 3:22 PM on July 3, 2006

With an old-style electromechanical meter, you are indeed being charged for the energy used up by the running of that meter. But, by design, it doesn't take very much power at all to run one of those things; the cost might be measurable in cents per century.

With the newer-style electronic meters, I'm not sure whether the power supply for the meter itself comes from your side or the utility's side; but considering that the power required is of the same order as that needed for a pocket calculator that can run for years on one tiny button-style battery, once again it really doesn't matter.
posted by flabdablet at 6:23 PM on July 3, 2006

PH has it slightly wrong, by the way. It's not energy that's voltage times current, but power.

The unit of power is the watt. One volt times one amp is one watt. So, a sixty watt light bulb being run off a 120V supply will be drawing half an amp.

A kilowatt is a thousand watts. Appliances in the kilowatt range usually have some kind of heating function: bar radiators, clothes dryers, electric kettles etc.

Energy is voltage times current times time. The official SI unit for energy is the joule - one watt being consumed for one second. More commonly seen on household electricity bills is the kilowatt-hour - one thousand watts being consumed for one hour. A kilowatt-hour is 3600 joules, for what that's worth.

You will occasionally see people who speak of "kilowatts per hour". Ignore them; they are confused.

If you run a two-kilowatt heater for one hour, you've used two kilowatt-hours of energy.

You're charged for total energy use. It doesn't matter to the electricity company whether you use your kilowatt-hours slowly (by running one small lamp for a long time) or quickly (by running your huge air conditioner for half an hour); it's the total number of kilowatt-hours you use in a given billing period that costs you.
posted by flabdablet at 6:36 PM on July 3, 2006

Sorry, I have it slightly wrong too. A kilowatt-hour is 3,600 kilojoules (3,600,000 joules, or 3.6 megajoules).
posted by flabdablet at 6:38 PM on July 3, 2006

to expand on flabdablet's comment, currently you are not charged as to when you use your kwh, but this is slowly changing. Utilities are rolling out meters which not only measure how much, but when you use your power, and varying billing rates accordingly. This encourages consumer use outside of peak hours (when industry power draw drops / ac use drops etc.) which means less invested in peak power generators. Most large industrial consumers are already billed this way, with different rates for different parts of the day.
posted by defcom1 at 7:00 PM on July 3, 2006

A kilowatt is a thousand watts. Appliances in the kilowatt range usually have some kind of heating function: bar radiators, clothes dryers, electric kettles etc.

Or Pentium 4 CPUs, which pretty much have a heating function too :P (okay, okay, not a kW, but you could fry an egg!)

You're charged for total energy use. It doesn't matter to the electricity company whether you use your kilowatt-hours slowly (by running one small lamp for a long time) or quickly (by running your huge air conditioner for half an hour); it's the total number of kilowatt-hours you use in a given billing period that costs you.

Further to defcom1, large organizations are also billed for peak demand, which is the peak kW load you put on the system over the billing period - so, it does matter when you use stuff to the electric company if you are really big. It is unlikely that residential customers will ever see that kind of billing exactly. You do pay a different fixed cost for installing 60A, 100A, or 200A service to your house. I guess the electric company could add a monthly fee based on the capacity of your service - not done now (I don't think), but it could come one day.
posted by Chuckles at 8:28 PM on July 3, 2006

Plenty of places already have domestic tariff systems that charge less for electricity used at different times. In the UK for example, Economy 7 is available to many domestic consumers, allowing much cheaper electricity for a 7 hour period during the night.
One thing that you might see in the future is increased provision of energy services offering to meet the full range of consumer energy needs rather than just acting as suppliers. Some services are already available but there is considerable potential for growth and for potential benefits in terms of reduced energy use/costs.
posted by biffa at 3:58 AM on July 10, 2006