There’s a couple of configs you can use.
“Extra power” can mean 63A breaker - house ~= 20A / 10A / 50A available over time. In that case, your car will charge as fast as it can without tripping your main breaker. (63A => 13.8kW, cars can easily charge at 7kW, not leaving a lot left over.)
If you have solar, “extra power” can mean exactly what you said.
Remember that most grid-tied systems over the world used to just feed everything back. That was the major reason you installed it, saving electricity, but as importantly, getting paid for your feedback. Scaling back production is relatively new. In AUS (some states or all, not sure) there is now a requirement that feed-in be limitable by the utility itself as operational needs arise, probably via a ripple device, such as we use for geysers. Your grid-tied inverter will have an input which will control feed-in. I’m unsure how exactly it works, but could be a 0-10V input or 4-20ma input which then controls the output based on setup params, similar to the car PWM mentioned above. (Not to go on too far a tangent, but these are interesting too: the cheap inverters just scale back their own production blindly, the better ones measure at the main breaker and scale that point.)
So, given ^, in most places “extra power” will mean anything extra produced by solar, which the harvi will measure and tell the zappi to consume. “Keep this point at zero” applying to the main breaker.
In SA, where you’re probably not pushing back to the grid at all, i.e. we rather scale back, it becomes tricky. Putting a harvi and a ET112 next to each other will just make the harvi never see the extra power, as it scales down before it can be seen. If you could have them offset a bit, “fighting” each other, that could work, but would be cumbersome.
Coming back to the start of this thread, IF you had an EVSE you could externally control (via RS485 / Modbus / Wifi / etc) AND you could determine “extra” yourself, you could do the same.
But a Victron-blessed EVSE makes that a lot easier, obviously. Zappi is very popular in the UK (maybe rest of Europe too?), so a first-party integration might be possible to Victron equipment, similar to the Fronius integration. Or the Victron-made EVSE does exactly this, but options are nice.
(FWIW, the Fronius integration is so strong, that the companies have a support agreement with each other: both Victron and Fronius will provide support for a combined system themselves, without first blaming the other guy. Pretty sweet for the customer.)
