Robert, I’m shocked you don’t know - you are the font of all knowledge on here…
BS EN 50470-1 is the standard that meters have to be built to and there are three levels of accuracy. No mention of a minimum unit of measurement though.
Interestingly, wikipedia on electricity meters says:
The accuracy is generally laid down in statute for the location in which the meter is installed. Statutory provisions may also specify a procedure to be followed should the accuracy be disputed.
The UK gov site isn’t very helpful, talks about accuracy and testing etc but not about a minimum unit of measurement, although there’s lots of mention of watt hour meters (1000imp/kWh) although we know that some are now more accurate (4000imp/kWh) → National regulation: gas and electricity meters - GOV.UK
For the United Kingdom, any installed electricity meter is required to accurately record the consumed energy, but it is permitted to under-read by 3.5%, or over-read by 2.5%. Disputed meters are initially verified with a check meter operating alongside the disputed meter. The final resort is for the disputed meter to be fully tested both in the installed location and at a specialist calibration laboratory. Approximately 93% of disputed meters are found to be operating satisfactorily. A refund of electricity paid for, but not consumed (but not vice versa) will only be made if the laboratory is able to estimate how long the meter has been misregistering. This contrasts with gas meters where if a meter is found to be under reading, it is assumed that it has under read for as long as the consumer has had a gas supply through it.
So never query a faulty gas meter!!
I guess we could set up a test on a smart meter?