Electricity Meters

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Working out Power Consumption

   

The humble electricity meter found on most premises can provide some useful information.

Working with the meters:

1/ Identify the type(s) of meters installed

There are a few different types of meters around the place. All keep a running total of kWh consumed, but there are a couple of different ways of displaying the data. I've heard of one type that doesn't have a display on it - It requires the meter reader to interrogate it with a special computerised device. Fortunately, most meters are user readable.
Most meters also provide a means of determining instantaneous consumption (The amount of power being consumed at the instant the meter is read). The two broad categories of meters covered on this page are Rotating-disc meters and digital meters.

2/ If there is more than one meter installed, the next step is to identify the purpose of each one.

Premises which have a larger electric hot water system installed often have this seperately metered with tariffs charged at a cheaper off-peak rate. Note: The digital meter installed at my house contains both a domestic and an off-peak meter and control circuit, with a switch to select which reading is displayed.
On some premises there are more than one 'normal' or domestic meter. The first thing to do is to work out what each meter is measuring. Your electricity bill may come in handy here - If you have off-peak, it will be listed seperately. The serial numbers of the various meters and the last readings taken marked on the bill should start you in the right direction. Also, by turning most appliances off then switching things on individually (especially large loads like heaters) and observing the rotating disc on an analogue meter or the flashing LED on a digital it should be possible to work out which meters feed the various circuits.

1....Rotating Disc Meters

The 'rotating disc' type of analogue meter has a disc located near the centre of the unit which can be seen as it rotates in the horizontal plane. As well as displaying accumulated kWh, instantaneous consumption can be calculated by timing how long this disc takes to rotate through a given number of divisions or revolutions.

There are 2 main ways of displaying the accumulated kWh.:

Odometer style readout: The newer meters normally have an easily readable display which is similar to a car's odometer. Digits are read from left to right.
'Clockface' or 'dial' type: These older meters aren't quite as easy to read at first, but with a little practice are fairly straightforward. The following web pages have instructions on reading this type of meter. Powercor, Western Power.

 

Measuring Watt-Hours (Pt):

   

Method:

Advantages :
Disadvantages :
  • Have to turn everything else on that circuit off - and remember to turn them on again when you've finished!
  • Only really applicable for large loads like heaters and air-conditioners.
  • Accuracy is limited to 0.1 or 0.2 kWh.

 

Deriving instantaneous power (Pi):

   

This method involves timing how long the spinning disc on the meter takes to rotate through a given number of divisions or rotations. The formula given below can be used:

Pi = (3600 * N) / (T * R)

where:

   
Pi = Real power being used at that point in time in kW   T = Time (in seconds) for the disc to rotate through the N rotations or part of a rotation
N = The Number of full rotations counted. When measuring smaller loads, it's more appropriate to calculate this as a fraction of a rotation. Normally one rotation consists of 100 divisions.   R = The number of revolutions per Kilowatt hour (rev/kWh) of the meter being used. This is normally printed on the meter. A few values I've seen are 133.3, 266.6 & 400.

 

Example:
  Calculate the power being consumed in the house at present if the meter takes 2 min 15 sec (135 seconds) to rotate through 50 divisions, using a meter with a rev/kWh value of 133.3, and total number of divisions per revolution of 100.

P = (3600 * 50/100) / (135 * 133.3) = 0.1 KW or 100 Watts.

Notes:

Advantages :
  • Can be used to measure consumption of appliances that are hard wired.
  • Good for measuring devices with constant load.
Disadvantages :
  • Have to turn everything else on that circuit off - and remember to turn them on again when you've finished!
  • Any readings taken will be for instantaneous consumption, so may not give an accurate reading for varying loads.

 

2....Digital Meters

These meters may measure consumption on more than one circuit at a time, in which case a switch is used to switch between the various displays.

The digital meter that I have on one of my circuits is a lot more limiting than the equivalent rotating-disc meters for this type of application. The kWh resolution is to whole units and not to the tenths of units which can be read from other meters, and the only way to measure instantaneous power (Pi) is to keep watching a small light (LED) on the meter which flashes for every Wh of power used. The time between flashes could be noted, and calculations outlined in the example below performed to come up with a figure for Pi.

Example:
  With only a large-screen TV operating in the house, the light on the digital meter flashes once every 24 seconds. Assuming the load is constant, what is Pi and how much power will the TV have consumed in 2 hours ?

There are 3600 seconds in an hour, so the TV would have consumed 3600/24 = 150 Wh in one hour. Therefore, Pi = 150 W, and it will have consumed 150*2 = 300 Wh of power in 2 hours.


Last Updated: 31/05/04