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I recently compared my inverter's built in generation meter with the external one for the FIT and found that there is a discrepancy of 515 KWh between the two. The inverter meter is reading higher than the FIT meter.

The fit meter is registering 27.3MWh and the inverter registers as 27.8MWh

Would this be normal or is this difference too high?

Just interested to know if this is normal and if not what the cause of the issue is (as I recently installed batteries I am hoping this is not causing the issue).

I have never done this comparison before so I don't know if this is recently an issue or if the two meters have drifted apart over the last 6.5 years?

thanks,

cojmh wrote: ↑Fri Sep 16, 2022 3:32 pm I recently compared my inverter's built in generation meter with the external one for the FIT and found that there is a discrepancy of 515 KWh between the two. The inverter meter is reading higher than the FIT meter.

The fit meter is registering 27.3MWh and the inverter registers as 27.8MWh

Would this be normal or is this difference too high?

Just interested to know if this is normal and if not what the cause of the issue is (as I recently installed batteries I am hoping this is not causing the issue).

I have never done this comparison before so I don't know if this is recently an issue or if the two meters have drifted apart over the last 6.5 years?

thanks,

I'm sure that discrepancy would be within accuracy tolerance limits.

Most meters have a tolerance %.

Most meters are calibrated a bit roughly.

If you want a calibrated meter, and get it certified i find ELTIME CONTROLS UK a very helpful company, but their stuff is costly.

I have calibrated high precision Frequency and DVM meters and mechanical reference amp meters, all HP Lab instruments, and they need supposedly to be calibrated every year with a paper trail as long as your arm.

3.2 Electricity meters are considered “accurate” if the permitted margins of error do not exceed +2.5% to -3.5% throughout the entire load range at which the meter is designed to operate.

Taken from: https://assets.publishing.service.gov.u ... 3-2014.pdf

So if the inverter is correct the external meter is about 2% low, but if the external meter is correct you inverter is about 2% high. How you wrk out which, if either, is correct I don't know.

I looked into this a few years ago, as I have metering on several circuits and I found that if I added the readings up from all of them it was a fair bit out from the electricity meter on the supply. The standard for a normal domestic meter used for billing is an error of +2.5% and -3.5%. This means that for an actual energy used of 24MWh a meter could read between 24.6MWh and 23.16MWh (a difference of 1.44MWh!) and it would still be within the allowable tolerance.

The metering built in to inverters doesn't have to be as good as this, as they don't have to comply with the billing accuracy requirement, so I suspect it's quite likely that the tolerance will be greater, perhaps around + or - 5%.

I will admit to being surprised that meters used for billing had such a high allowable error, I'd assumed they would be accurate to around 0.5% or better.

Thanks for all the answers and the links.

So it seems that everything is within the allowable tolerances (although like OGB I am surprised at the allowable limits - I guess this is down to the technical capabilities of meters at a particular price point).

There is also the cable losses from the Inverter to the meter (plus some internals of the inverter itself) so I would always expect the generation meter to be a little lower even if the accuracy of both meters was identical.