Sorry, not true.
After a while the Riemann values get huge and the results in the utility meter are just rubbish.
I guess, that is what we call ‘small differences of large numbers’.
What do you mean by this?
Here’s an grafical explanation how it works (sensor + utility_meter).
If you use a meter that does not deliver kWh but Watt. You need an integral sensor on the power sensor that calculates the energy kWh out of the power W.
Its german, i hope google translate will help.