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# Flashing LED On Electricity Meter

## Find out how to interpret the flashing LED light on electronic electricity meters

education | general | solar

If you have had an electricity meter installed in the last few years, and certainly if you taken advantage of the feed in tariff to have a solar PV installation, your electricity meter(s) will be electronic with an LED which flashes. The rate of this flashing is proportional to the amount of power passing through the meter, and so useful information is there to be collected.

Pictured above as an example is an A100C single phase kWh meter which is commonly installed in the UK. To the right hand side of the meter reading is a small LED together with some writing as pictured below:

This type of meter will always be labelled in this way with a certain number of Imp/kWh. Imp/kWh is short for Impressions per kWh (unit) of electricity which passes through the meter where one 'impression' is a brief flash of an LED.

The majority of meters are labelled 1000 Imp/kWh which means that the LED will flash 1000 times for each kWh of electricty which passes through. The rate of the flashing of the LED tells you how much power is currently passing through the meter.

For the meter in the photographs above, the label states 1000 Imp/kWh, so the LED will flash 1000 times per unit of electricity. We can use that value of 1000 to estimate the power. If a constant 1000W (1kW) is going through the meter, then in one hour (3600 seconds) the LED will flash 1000 times, so the LED will flash once every 3600/1000 = 3.6 seconds. With a constant 3000W (3kW) going through the meter, in one hour 3 kWh will pass through the meter (3 x 1000 = 3000 flashes in 3600 seconds) so the LED will flash every 3600 / 3000 = 1.2 seconds.

It is more likely that you will want to measure the rate of flashing, and from that calculate the power going through the meter. This can easily be achieved by reversing the calculation above as follows:
Power (kW) = 3600 (secs in 1hr) divided by (the seconds between flashes * number of Imp/kWh printed on meter)

For example, if the LED flashes once every 5.2 seconds on a meter labelled 800 kWh, the power going through the meter at that time will be 3600 / (5.2 * 800) = 0.865 kW.

(To measure the number of seconds between flashes it is best to start timing on a flash, and then time how long for 10 more flashes. Divide that time by 10 to get the average number of seconds per flash. Obviously this only works if the power is constant - if someone turns on the kettle half way through your time measurement the results will not be valid.)

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Page Last Updated on 19th May 2013 at 08:29:13pm

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