#1 June 15, 2015 19:13:41

muymalestado
Registered: 2011-12-23
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Smart meters and 'meter accuracy'

My searching techniques didn't show up likely previous discussions - and - there are two queries here. Prior apologies.

Smart meters are mentioned / threatened on all sides these days. My impression of them is negative thus far, but I have not yet laid eyes on one.

Do we really need thousands of neighbourhood transceiver towers to mimic what the internet now does? Why do all the smart meters require an always-connected to every user network?

As a FIT recipient I now receive payments at irregular intervals; after my reporting to their timetable. The amount paid out seems to be at the latest FIT rate. Latest in that at 1st April there is usually an increase. Smart meters might let the FIT payer know each month what my generation is and receipts would then be reduced appropriately for each month preceding 1st April.

Meters again but this query is on accuracy.

10,000kWh has just been passed - hooray - on the generation meter. The inverter shows 10,323kWh.

Did I lose 323kWh or is this about the level of accuracy one might expect?

Thanks.
Phil

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#2 June 16, 2015 08:59:54

bhommels
Registered: 2011-10-03
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Smart meters and 'meter accuracy'

Hi Phil,
For my inverter (SMA) the manual states the inverter internal generation meter is not a calibrated device and should not be relied upon for measurements. The separate generation meter is calibrated and certified, so that is the one that gives the ‘true’ readings.
My inverter lags the generation meter by ~1.3% on average. Your case of 3.2% sound a bit much, but in reality the inverter manufacturer has no obligation to make the internal meter accurate to any standard.
In my case, the inaccuracy is larger for low power levels, and given the craggy waveforms put out by the inverter it makes me think the disagreement between the two meters stems from the power factor estimation.
Regards, Bart

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#3 June 16, 2015 11:10:24

muymalestado
Registered: 2011-12-23
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Smart meters and 'meter accuracy'

Well Bart - thanks, that sounds about what I thought and hoped for. Ie., that the 3.2% was not ‘lost’. The wording in this inverter manual lacks the definite explanation you gave.

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#4 June 16, 2015 16:31:10

Adamthesolarman
Registered: 2015-06-16
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Smart meters and 'meter accuracy'

The difference from the readings on the inverter and export meter can be affected by the length of the cable run and whether the cable is copper or aluminium.


Energy losses in a cable is mainly due to resistance heating of the cable.
It is given by the following formula :
E = a x R x Ib²
Where :

E : energy losses in wires, Watt (W)
a : number of line coefficient, a=1 for single line, a = 3 for 3-phase circuit.
R : resistance of one active line
Ib : current in Ampere (A)

R is given by the next formula :
R = b x ?1 x L / S

b : length cable factor, b=2 for single phase wiring, b=1 for three-phased wiring.
?1 : resistance of the material conductor, 0.023 for copper and 0.037 for aluminium (ambient temperature = 25°C) in ohm.mm²/m.
L : simple length of the cable (distance between the source and the appliance), in metres (m).
S : cross section of the cable in mm²
NB : for direct current the energy losses in percent is equal to the voltage drop in percent.

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#5 June 18, 2015 09:50:39

bhommels
Registered: 2011-10-03
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Smart meters and 'meter accuracy'

@Adam: this is relating to the cable between inverter and generation meter, which is usually short (10 cm in my case).
For the sake of the argument, let's assume 10m of 1.5mm2 single phase copper cable between a 4kW inverter running at full whack (16A), and the generation meter. Plugging in your numbers, this results in a worst-case power loss of 2%. More than I (handwavingly) expected, but not enough to explain the 3.2% loss - or my 1.3% gain!

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