Where Problems Arise
It's typical for an EV to draw up to 7kW when charging. However, a 2kW lower limit is commonly set to make hardware efficient and keep losses low.
Solar Output and Battery Status: Imagine your solar system is generating 5kW of electricity. At this point, your home battery is fully charged, and the household is consuming only 1kW.
EVSE in Action: Your EVSE (the charger) then harvests the remaining 4kW that would otherwise be sent back to the grid.
Changing Household Load: Now, you switch on a 2kW kettle and the household load rises to 3kW.
EV Charging Adjusts: The EV charging rate dips to 2kW to match the available solar yield.
At this point the solar yield matches the total consumption
Solar Output Decline: A cloud passes, causing the solar yield to drop by 1kW, leaving you with a 1kW energy shortfall.
Where It Goes Pear Shaped:
- The home battery, designed to bridge such gaps, kicks in to supply the extra 1kW.
- Oblivious that this load is not essential household use, home battery keeps up supply to the EVSE
- Ultimately, your home battery, thinking it’s fulfilling its role, empties itself into the car.
By walking through this scenario, you can see how your home battery and EV charger may not always be on the same page, leading to less-than-ideal outcomes. If you like Irish accents and watching a bloke hitch his pants up, slog through this youtube explainer, the useful part is between minutes 3:50 & 14:30.
The Solution Is To Create A Pecking Order
Unless there's a war, bushfire, flood or virulent plague, chances are you're never going to want to charge your car with your house battery anyway. So the simple way to fix the problem is to put the EVSE outside the house so to speak.
The smart EV charger uses a CT to monitor and divert solar export. Installing that CT and connecting the actual charger to the mains upstream of the battery/solar consumption meter, means the house battery will never see the EVSE. It simply doesn't know it exists and will not discharge the battery to meet the load.
The charger will still monitor for export but it'll get second bite at the solar yield, the house battery will be charged first (unless you can somehow program the solar/battery inverter to delay it's charging with a schedule?) However this scenario, filing the small tank first, is perfectly rational.