Monitoring solar output and household consumption.

[Doggieman]

Dear all,

I'm totally new to domoticz and I would like to start using it. But I'm not sure if it is the way to go for me. This is the situation:

I have a 5kw solar installation on my roof. I'm monitoring that instalation with a raspberry-pi and upload the values to PV output. org. I'm very happy with this setup and it works flawlessly for more than a year now.
I also have a old fashion ferrarismeter that runs backwards if my solar instalation is producing more kilowatts then the households consumes. The situation is that I would like to monitor with domoticz when my meter is running backwards, and how much. In other words:if I deliver power to the grid then I would like to switch on my electric water heater. This water heater has multiple 500 watt electric elements inside it so if possible I would like to switch on more elements if the solar instalation is pushing more energy into the grid. I'm not a programmer but blocky in domoticz looks easy enough to learn.
The problem is how do I detect that mij meter is running backwards and how "fast" in or with domoticz.?

Let me explain: I could use two cent - am-meter devices to monitor the solar instalation and the house consumption, but where to put that one? My solar has is dedicated connection in the switchboard so I can measure the amount of power it's producing. No other things connected to that connection. But it will run thru the ferraris meter eventually. So how to tell for sure if I'm using or delivering power to the grid, and how much at that specific moment?