# Unit Of Measure For My Electricity Meter

1. Jul 18, 2009

### DaleSwanson

So I was trying to figure out how much power items at my house use. I have a standard looking power meter outside with the dial http://en.wikipedia.org/wiki/File:Electrical_meter.jpg" [Broken]. I am trying to figure out though what unit the meter uses. My first instinct was watt hour, but I can't get the numbers to match.

The dial has numbers on it 0 - 90, so a full rotation is 100 units of something. It takes about 40 seconds to make a full rotation mostly everything off (like if we weren't home), and 16 seconds to make a rotation with a lot of stuff on. That works out to 90 and 225 rotations per hour. This should give a nice lower and upper bound. Last month our bill was 696 kWh, which works out to 967 watts on average.

If I assume one rotation is 100 watt hours then the monthly usage would work out to 6,480 kWh and 16,200 kWh, way too high. My next thought was that a full rotation was only one Wh. This would give us 64.8 kWh and 162 kWh, way too low. It seems be off by a factor of 6. Then I thought the meter said 240v, so it probably meters at 240 volts, if the meter did 240 volts and bill was in 120 volts that still wouldn't be right, plus I think that's pretty unlikely. I also noticed the meter says 240v 3W, so if each turn was 3Wh at 240v, and the bill was in Wh at 120v that would actually work out, but that's a pretty big stretch.

So, does anyone know how the dial on the meter measures usage?

Last edited by a moderator: May 4, 2017
2. Jul 18, 2009

### negitron

It tells you right on the meter. That figure that says Kh 7.2 is the number of watt-hours used per disc revolution.

3. Jul 18, 2009

### DaleSwanson

Ah, I see. Mine also says 7.2, so I guess my new bounds are 466 kWh and 1166 kWh, which seems right. I must say, it seems like a strange thing to do, although I suppose there is a reason behind it.

Thanks.

4. Jul 18, 2009

### negitron

Strange in what way?

5. Jul 18, 2009

### DaleSwanson

Where does the 7.2 come from? Why not simply use 1 turn = 1 Wh?

6. Jul 18, 2009

### Pumblechook

You should get a clip-on ammeter.

7. Jul 18, 2009

### negitron

An ammeter alone will not tell you your power consumption. You need to know the current, the voltage and the power factor for an accurate reading.

8. Jul 18, 2009

### vk6kro

The main dial is marked in KWH so at present the one in the picture is showing 86174 KWH

Run something for an hour and see how much the main meter reading changes.

The 3 watts might be the power the meter itself uses.

You can get plug in digital power meters that give a readout of Volts, Amps, Power factor, Power, KWH and KVAH and total cost.
So, you can test something like a refrigerator (which is turning on and off) over a week or so to see what it costs to run the fridge.