And here is Tesla's reply. It rather confirms my suspicions about reactive power metering - If you have Powerwall, don’t get a smart meter installed!
I like the beer glass analogy. The only problem is that during the day my glass is almost empty!
"Thank you for your reply.
From our investigation we have noticed that most of the times you only import energy from the grid overnight and since the solar is really poor the battery usually offsets your house loads during the peak hours.
I suspect that this discrepancy is due to the fact that your meter reads also the ‘reactive power’ while the App does not. The app shows only the ‘real power’.
To make this difference clear here it is a good and easy example:
The App only shows the Real power while I suspect your meter shows also the reactive power.
You can also check online the specific differences between the two types of “power”.
By the way, the system does not seems to record any import from the grid except the tolerated 0.4/0.5kW that basically is the amount of energy that the inside inverter uses to keep the modules DC to AC and AC to DC ready to be used.
I also suggest you to contact your energy supplier to check the meter and the installer of the battery to maybe do some checks on the CT clamp positioned on the site side.
I will anyway escalate the case to our Tech. Support to let the double check because I do not see this data discrepancy but maybe they can help me."